Ronald Reagan began his political life as a 21-year-old follower of Democrat Franklin Delano Roosevelt. Now there are sober people who think Reagan, at age 69, might become the FDR of a born-again Republican Party.

As delegates gather here for the formal Republican National Convention opening Monday, confidence in Reagan's beating Jimmy Carter has soared to the point that some are beginning to talk of 1980 as a breakthrough year for the GOP -- the kind of turning point in American politics that Roosevelt's election in 1932 turned out to be.

Robert Teeter, the Detroit pollster whose recent studies have fueled hopes for a top-to-bottom Republican victory, said Friday, "I think it is possible for Ronald Reagan to build a coalition that would last more than one election and even reshape our politics on a long-term basis."

Richard Wirthlin, Reagan's pollster, took a more cautious line, saying, "This could become a watershed year, but it isn't as of now. There are some below-the-surface indicators, however, that augur well for the Republicans. It's a question of how they develop."

They could, of course, run themselves right into the ground. Reagan's current lead over Carter -- 39 to 31 in Teeter's mid-June poll survey, with 21 percent for independent John B. Anderson -- is no guarantee of victory over an incumbent. Carter himself led Gerald R. Ford by three times as much at the time of his nomination in 1976 and barely held on to win.

And even if Reagan did reclaim the White House for Republicans by a wide margin, his victory might be like Dwight D. Eisenhower's in 1956 and Richard M. Nixon's in 1972 -- a "lonely landslide" that did nothing to disturb the Democrats' healthy majorities in the House and Senate.

Still, enough differences exist between 1980 and those earlier Republican years to make the possibility of a landmark election a matter of lively debate among politicians, pollsters and students of American government.

The most important difference is the degree of disarray and disunity among the reigning Democrats. In 1956, Adlai E. Stevenson was a retread candidate, with scant prospect of beating the popular Ike, but the Democrats, if downcast, were not seriously divided.

In 1972, George McGovern's nomination was very controversial; the AFL-CIO and a great many elected Democratic officials took a walk on the presidential candidate. But the most emotional issues were social and cultural and some areas of foreign policy -- not the heart of the economic program that was the glue for the coalition that Roosevelt began assembling in 1932.

"Reagan's opposition to abortion and to the Equal Rights Amendment and his vigorous condemnations of atheistic communism are a badge of honor" to many of the Democrats who defected during the McGovern run, said University of Virginia political scientist James Ceaser. Today, therefore, "One may reasonably ask why these voters would shift permanently away from the Democratic Party since they did not do so during the Nixon years."

"The crucial change," he wrote in a Los Angeles Times article last April, "is that the pull of economic issues for the Democrats could be neutralized or even turned to the advantage of the Republicans. And this, in turn, would allow voters, without any feelings of being cross-pressured, to swing to the Republican side."

Teeter's survey for the Republican National Committee and the Republican Congressional Campaign Committee suggested strongly that the combined impact of inflation and recession is, as Ceaser suggested, breaking the bonds of loyalty in the Roosevelt coalition.

Teeter reported that in the last six months, Republicans have moved from an 8-point lead to a 29-point lead as the party that would do a better job of controlling inflation. In the same six months, the GOP erased 18 points of a 21-point deficit as the party that would most likely reduce unemployment. Republicans are now nearly even with the Democrats on the issue that has fueled Democratic victories ever since 1932.

The Republican platform drives that argument home, saying that "the Democratic Congress and the Carter administration are epousing programs that candidate Carter in 1976 said were inhumane: using recession, unemployment, high interest rates and high taxes to fight inflation . . . a bankrupt policy which is throwing millions of Americans out of work . . . We categorically reject this approach."

On Friday, Republican leaders here met with Douglas Fraser, president of the politically potent United Auto Workers union, which has 30 percent unemployment in its membership. "People of my generation have been running against Herbert Hoover since 1932," the 62-year-old Fraser, an ardent Democrat, said at the end of the meeting. "And now in 1980 we find they [the Republicans] have the issues and we don't."

During the primaries, Reagan claimed he was establishing an appeal to blue-collar voters and other elements of the Roosevelt coalition. The claim is somewhat questionable. In two major "crossover states," Illinois and Wisconsin, Reagan got only about one-fifth of the Democrats who voted in the GOP primary, trailing Anderson in both states and George Bush in Wisconsin.

But there is no doubt that Republicans are going after traditional Democratic voting blocs more seriously this year than they have in decades. The platform includes an unprecedented section promising black Americans that Republicans will stand "shoulder to shoulder" with them in pursuit of equal rights and economic progress. A black Republican leader from Maryland, Dr. Aris Allen, is the convention secretary who will call the roll on Reagan's nomination.

There are major programs in the platform for the elderly and for urban dwellers, and a strong pro-Israel foreign policy plank. Most of all, as party chairman Bill Brock says, there is an emphasis on "jobs, jobs, jobs."

If Republicans have finally escaped the stigma of Herbert Hoover and the Great Depression, then it is possible that a political epoch has ended. There are other, more immediate factors that indicate the extent of the Democratic vulnerability and the potential for a Republican breakthrough.

Despite his incumbency, Carter has not been able to close off the challenge from his rival, Sen. Edward M. Kennedy. Kennedy's reiterated claims that the president is a crypto-Democrat running on a platform that ignores the party's liberal heritage are viewed by Carter strategists as undercutting his credibility with traditional Democratic voters.

The Democrats' real power base for a generation has been Capitol Hill, but today, the Democratic Congress gets its lowest ratings from the public. A Harris Poll this spring found voters giving Congress a 70-to-26 negative rating, and Teeter's June study found that by 3 to 1 -- 32 percent to 11 percent -- voters thought the country would be better; not worse, if Republicans regained a congressional majority for the first time in 26 years.

Few Republican leaders are optimistic enough to predict actual GOP control of the House or Senate in November, but a weakening of the Democratic majorities is very likely.

And that, in turn, could spell bad news for Carter, because he trailed his party's ticket in 1976 and figures to do so again. Four years ago, Carter ran behind 281 of the 292 Democrats elected to the House.

As evidence of his unsettled relationship with Congress, last month Carter became the first Democrat since Harry Truman to have a veto overridden by a Congress of his own party, when the House and Senate overwhelming rejected his plan to raise oil taxes as a conservation measure.

The dissension between Democratic president and Democratic Congress has been matched by the frequency of disagreement among congressional Democrats themselves. Energy has been a particularly thorny problem area for the leadership in Congress, but congressional Democrats have also wrangled over tax rebates, Social Security tax rollbacks, defense cuts, balanced budgets and a dozen other subjects.

Teeter said the fact that "everything in Washington is run by Democrats now," which was not the case in 1956 or 1972, "means that voters who want a change -- and a lot of them do -- have to vote Republican."

Bill Sweeney, director of the Democratic Congressional Campaign Committee, put the same thought in different words, saying "voters may be ready for one party with one voice, rather than one party with many voices."

While the preconvention week here was marked by some intense and emotional debates on issues like the Equal Rights Amendment and abortion, Republicans have striven to present a picture of unity on the key economic issues they hope to use to crack the Roosevelt coalition.

An across-the board tax cut, described as a stimulus for investment and creating jobs, was endorsed jointly two weeks ago by Reagan and Republican congressional leaders, setting off a stampede by congressional Democrats to come up with an alternative.

Reagan and the congressional Republicans also have announced plans to lift the Carter-imposed embargo on grain sales to the Soviet Union -- a bid to bring farmers into a new Republican coalition.

Back last winter, before the nominee was chosen, the national GOP launched an unprecedented $5 million advertising campaign attacking Democratic control of Congress and urging, "Vote Republican -- for a change."

The effort is supposed to climax in the fall when Reagan, Republican members of Congress and scores of congressional candidates gather on the steps of the U.S. Capitol to sign a "contract" of basic laws they promise to pass in 1981 -- including their tax cut -- if voters elect Reagan and a Republican Congress.

Such party-line voting has been fading over the last 25 years of individual candidates for the White House, Congress and other offices have built their own precinct organizations, run their own advertising campaigns and targeted their own constituencies. The old notion of "presidential coattails" carrying candidates into Congress has become less and less applicable.

But the Reagan campaign is attempting to revive it -- within limits. Wanting to attract independents and ticket-splitters, Reagan is aware of "the risks of appearing too partisan," a top strategist said this week. But added, "If Republicans don't pick up some seats in Congress, Reagan's not going to be a very effective president, so he has an interest in promoting a party victory."

Teeter said in the interview that, "If we keep the election focused on one or two issues, like inflation and unemployment, then people may be driven to vote against the Democrats. If it gets to be seven or eight issues, then it gets to be complex and confused, and name-familiarity will be the anchor for the voters, as usual."

In fact, some unpublished sections of Teeter's poll suggest that Republicans would have trouble converting a top-of-the-line ticket victory into a party-side sweep.

Public attitudes are ambivalent. While 52 percent of those interviewed agreed with the statement that "Democrats have controlled Congress so long that we might be better off if we give the Republicans control for a while," a larger 71 percent agreed with the somewhat contrary statement that "even if the voters elected mostly new people to Congress, it probably wouldn't change things much."

Despite the GOP advertising campaign, the link between presidential and congressional voting decisions is closer for Democrats than for Republicans. By 73 to 19, Carter voters in Teeter's poll said they planned to vote for a Democratic congressional candidate, while 69 percent of the Reagan voters said they were planning to vote Republican for Congress and 23 percent were planning to vote Democratic.

The Teeter poll also indicated that Republican leaders have reason to fear that John Anderson's candidacy may hurt their chances for congressional gains. By 51 to 31, the Anderson voters said they planned to vote Democratic in congressional races.

Some Republicans think that Anderson's bid may eventually redound to the benefit of House Republican candidates, if voters become convinced that the independent candidacy may force the presidential election into the House of Representatives for decision. In almost every recent election, Republican presidential candidates have carried dozens more congressional districts than GOP House candidates have. If voters are persuaded that the only way they can be sure their presidential vote "counts" is also to vote Republican for the House, "coattails" could grow overnight, or so this argument goes.

While all of these analyses have at least a surface plausibility, there are more skeptics than believers -- both among academic students of American elections and, more surprisingly, among the Republican strategists.

Everett Carl Ladd Jr., University of Connecticut political scientist and student of public opinion, said of Teeter's data and other polls: "There's no new majority there; there's just evidence of the weakness of parties. What the voters are really saying is that they doubt anyone can make a difference on the things they care about."

James Sundquist of the Brookings Institution, who has written a book on political realignments, said, "I don't see 1980 as a critical election in the way that 1896 and 1932 were. In a critical election, you'd look for evidence of reattachment to the parties and a reduction in independents; I see little of that."

In fact, the data on party identification show great stability, not dramatic change. Gallup Poll surveys from April through June of this year showed 23 percent of the people identify themselves as Republicans, 47 percent Democrats and 30 percent Independents. It is hardly conductive to GOP euphoria that those are the identical percentages Gallup found in the period of July-October 1974, when Richard Nixon's forced resignation brought the Republican Party its greatest recent embarrassment.

Historical data show Republicans were in the 30-plus percentage range from the mid-1930s until 1960, in the high 20s until the start of Nixon's second term in 1973, and in the low 20s ever since. In the most recent Gallup data, Democrats outnumbered Republicans in every socioeconomic group, including those with $20,000-plus incomes and in professional-business jobs.

As Ladd and Sunquist's comments indicated, the only "growth party" has been the independents, who have gone from 16 percent in the mid-1930s to 30 percent in the most recent surveys.

The Teeter data did have one hopeful indicator for Republicans on the basic question fo party indentification -- an apparent shift among the independents toward a Republican commitment. The percentage of independents leaning Republican, basically steady at 7 to 8 percent since the early 1950s, jumped in June to 15 percent.

Reagan's pollster Richard Wirthlin said this finding was paralleled in his own recent data and suggested that "change is in the air." He also said "it's probable that party loyalties will follow, rather than presage, a massive change in voter support."

Wirthlin also said his own data clearly show that the voters' position on a basic liberal-conservative questions -- government responsibility for maintaining a high standard of living -- is much closer to the perceived position of the Republicans than of the Democrats.

His finding tended to reinforce the comment of Rep. David Stockman of Michigan, one of the young architects of the activist conservative positions reflected in the Republican platform, that you've got to believe people are looking for an alternative" to present Democrat policies.

Asked if that meant a swing to Republicans, Stockman smiled and said, "Hope springs eternal."

John Deardourff, a Republican campaign consultant who is heading one of the independent campaign efforts to aid Reagan, dismissed the talk of a permanent Reagan majority as wishful thinking.

"I think the blue-collar vote may go to Reagan in this election," Deardourff said, "but I don't think it will go permanently to the Republican Party, or go against incumbent Democrats to any great extent."

"I don't think you'll see a big Reagan sweep," he said, "and you'll see Reagan winning many places where Democratic incumbents also win. I don't see any permanent shifting taking place."

The final word in this debate came from Nelson Polsby, University of California author of a series of studies of presidential elections.

"Critical elections are things that happen retrospectively," Polsby said. "You don't know about them ahead of time. Public opinion is so volatile now that you have to take any one poll -- or set of polls -- cautiously. But even if a tidal shift were taking place, I don't think we could say so with assurance now."