Republican Party strategists have stressed for months that Maryland's Paul S. Sarbanes is the most vulnerable U.S. senator seeking reelection this year.

The contention -- based on some party polls and the defeat of several Democratic liberals in the 1980 election and Republican successes among traditionally Democratic blue-collar voters -- has led national Republican groups to label the U.S. Senate race in Maryland a top priority and to begin to pour money into the campaign of GOP candidate Lawrence J. Hogan.

Hogan's campaign, as well as a television advertising campaign launched 15 months ago by the National Conservative Political Action Committee (NCPAC), emphasizes the theme of Democratic vulnerability, with Hogan claiming that Sarbanes is beatable because voters view him as "a do-nothing senator."

But now, as the fall campaign gets under way, the once-edgy Democrats are pointing to signs of Sarbanes' growing strength around the state and saying the Maryland race could be a rout, particularly given Maryland voters' historic dislike of conservative Republican politicians.

One leading Democratic pollster who says he thought Sarbanes was on the "terminal list" 18 months ago now is confident Sarbanes will win.

"There is nothing in Maryland politics today that suggests that Republicans have much opportunity to do anything," said Leon Billings, director of the national Democratic senatorial campaign committee. "Right now Sarbanes is strong and getting stronger. It could be a blowout."

Democrats base their optimistic outlook on, among other things, what they see as a new political climate in 1982 that favors Democrats because of the electorate's concern over the economy and disillusionment with Reagan's programs. With unemployment in the state remaining at near-record high levels, the results from last week's primary as well as recent newspaper polls support that view, they say, by showing overwhelming approval of incumbent Democrats across the state.

"People are getting increasingly sensitized because of Reagan's policies," says Peter Marudas, a top Sarbanes aide. "Paul Sarbanes has said for two years that these policies are unfair. The economic picture is not good, and now people are very concerned. Voters who are supposedly apathetic are very interested this year."

In addition, Sarbanes' aides say, Maryland's junior senator will be running on a united Democratic ticket in a state that, with its 3-to-1 ratio of Democratic voters, was one of six that Reagan lost in 1980. This year, seven of Maryland's eight congressional races feature Democratic incumbents; Democratic Gov. Harry Hughes is a solid favorite to defeat his GOP challenger; and in two key districts Sarbanes will be aided by valuable political brokers, Mayor William Donald Schaefer in Baltimore (who appears in a Sarbanes television ad) and Baltimore County Executive Donald Hutchinson.

Sarbanes also is expected to do well in Montgomery County, where Democrats explain Reagan's 1980 success as a vote against former president Jimmy Carter rather than as a sign of a long-term conservative swing. Democrats are even predicting that Sarbanes will win Prince George's County, where Hogan is the county executive.

Hogan has predicated his ability to win state-wide as a Republican on his ability to get elected in the solidly Democratic, blue-collar areas. He and his campaign aides, unlike the Democrats, are convinced also that Maryland voters are conservative by nature and do not disapprove of Reaganomics.

But Sarbanes is counting on retaining overwhelming support from organized labor and blue-collar voters.

"The blue-collar vote is home," says Billings. "To the extent that it left [the Democratic Party] in 1980, it's back home."

With the Nov. 2 general election less than seven weeks away, Sarbanes will rely on a campaign strategy similar to the one that propelled him to the Senate in 1976: He will try to keep the Democrats united, retain his solid backing from organized labor, and avoid controversy. And he will continue to highlight his opposition to Reagan, while portraying Hogan as one of the president's staunchest supporters.

If there is a difference in this year's Senate race, which most observers expected to be nothing more than a classic liberal-conservative contest, it is the presence of NCPAC and its controversial ad campaign. The original NCPAC ads, which called Sarbanes "too liberal for Maryland," apparently backfired, drawing national attention to Sarbanes' race and enabling him to raise huge sums of money well ahead of the 1982 election.

Now NCPAC has resumed its ads -- this time they include pro-Hogan segments along with ones that are anti-Sarbanes -- and the Democrats seem determined to make political hay of it. Sarbanes campaign aides say NCPAC is "dictating some of the themes in this campaign" and "polluting the political process."

"Here is a group not based in Maryland and with no record of concern in Maryland," says Marudas. "It's really pernicious. They come in and have no accountability. They're political hit specialists."

Last week, in their opening salvo of the fall campaign, Sarbanes' aides charged that Hogan was "playing right off" NCPAC and using the group's rhetoric and tactics in his media campaign. Hogan and his aides maintain that they have had no contact with NCPAC officials, and they accused Sarbanes and the Democrats of trying to turn the ads into a campaign issue.

"The fact that we identified the same weaknesses in Sarbanes as NCPAC did is what leads to those conclusions [that there is cooperation between Hogan and NCPAC]," says George Nesterczuk, Hogan's campaign manager. "There is absolutely no coordination."

But Hogan and the Republicans plan to continue to stress their view that Sarbanes is vulnerable, citing his legislative record as evidence. "He has not had one of his initiatives passed into law," Hogan repeats to groups around the state. "He has been a very inactive senator. He's a very laid back person. I'm aggressive."

Some Democrats, including a leading pollster, say Sarbanes "has worked less hard than he might have in making his case" to voters.

Now, while Sarbanes aides dispute Hogan's contentions, they have begun a media campaign that is an obvious attempt to respond to some of the criticisms raised by Hogan and by NCPAC, particularly those suggesting that Sarbanes is a lackadaisical legislator. In his ads Sarbanes is portrayed as a behind-the-scenes negotiator respected for his integrity and hard work.

"The guy is no shrinking violet," says Marudas. "He's taken on [former secretary of state Alexander] Haig, [former president Richard M.] Nixon, [former national security affairs adviser Henry] Kissinger, and [former Maryland governor Spiro] Agnew."

Billings, of the senatorial committee, says Sarbanes is "a workhorse, not a show horse. The fact that he isn't flashy on Capitol Hill doesn't mean he isn't doing anything."

Sarbanes' final advantage, in addition to being an incumbent who won handily in 1976, is a campaign war chest that already totals $950,000. His aides say contributions continue to flow in steadily.

Hogan has raised $300,000 and is expected to benefit from an expenditure of $230,000 by the Republican senatorial campaign committee. In addition, Hogan has begun to raise money from conservative political action committees around the country.