The prospect of Sen. Edward M. Kennedy entering the Democratic presidential race has caused a fundamental change in Republican presidential politics.

Its immediate effect has been to overshadow the Republican contest. But more important, it has raised a round of questions about the ability of Ronald Reagan, the GOP frontrunner, to withstand a Kennedy challenge, and has led former president Gerald R. Ford seriously to consider entering the race.

A key element in Ford's decision to take "a serious look" at his presidential prospects is a feeling among his advisers that Ford would run stronger against Kennedy than would Reagan, who has not announced but who is considered by many a stronger candidate against President Carter. $5"Up until a couple of weeks ago, all the Republican candidates enjoyed a tremendous luxury," says David Keene, political director for the George Bush campaign. "There was a feeling that even our slowest horse could beat Jimmy Carter around the track. Therefore, the question of who could win didn't have to be answered.

"Now Republicans have to ask themselves which candidate has the best chance of winning against Kennedy. This tends to focus on everyone's negatives."

Keene thinks a Kennedy candidacy would hurt Reagan more than any other Republican. So do the political operatives for each of the other five major GOP candidates or probable candidates.

Each candidate offers his own rationale. John Connally, former Texas governor and former secretary of the treasury, cornered the early market in this regard. With typical immodesty, he's been claiming for months that he's the only Republican big enough, tough enough and dynamic enough to take on Kennedy.

"The more Kennedy emerges as a candidate, the more that helps John Connally," says Ed Mahe, Connally's campaign director. "Many people feel he will provide the most contrast to Kennedy."

There may, however, be more bravado than hard evidence behind Connally's claims. When a Gallup poll pitted Kennedy against GOP contenders last July, the Massachusetts Democrat beat Connally 2 to 1 in a trial heat.

Kennedy led Ford 54 to 41 percent, Reagan 56 to 8 percent and Tennessee Sen. Howard H. Baker Jr. 57 to 36 percent -- a surprisingly strong showing for Baker, an unannounced but probable candidate who trails Ford and Reagan by wide margins among GOP voters.

This kind of poll raises hopes in the moderate GOP camp. "Kennedy helps us. Right now Reagan beats Carter in all the polls, but he loses to Kennedy," says Ron McMahan, press secretary to Baker. "Against a Kennedy, Republicans will go for someone who is the most electable -- the candidate with the broadest appeal."

"When you're looking at an ideological candidate like Kennedy, a centrist is going to do better than another ideologue," says Keene, political director of the Bush campaign.(Bush barely registers on the polls against either Carter or Kennedy.)

"Connally has been playing his Kennedy card for months, saying he could beat him in a fight in a dark alley with short knives," Keene adds. "But you might want a contrast to Kennedy rather than someone who says they'll slug it out with him. The cool, reasonable approach of a George Bush might look pretty good."

This may be little more than wishful thinking, for the central fact of the early rounds of the 1980 Republican presidential nomination race is that Reagan has a wide, perhaps overpowering, lead over all of his potential opponents in terms of organization and personal popularity.

A Gallup Poll released today, for example, indicates that Ford was the only Republican even close to Reagan among independent and GOP voters surveyed. And Ford trailed Reagan, whom he narrowly defeated for the 1976 nomination, by 29 to 21 percnet.

Among Reagan's other opponents, Baker polled only 10 percent, Connally 8 percent and Bush 3 percnet. Rep. Philip M. Crane (Ill.), Rep. John Anderson (Ill.) and Sen. Bob Dole (Kan.), all declared candidates, didn't even register in the poll.

There is an understandable euphoria in the Reagan camp. "The governor will get the nomination unless we make a mistake," says Mike Deaver, one of Reagan's top advisers. "If we blow it, we have no one to blame but ourselves," says Sen. Paul Laxalt, Reagan's campaign chairman.

The irony about the Reagan compaign is that he has been able to hold the same, steady lead in the polls since March without becoming an announced candidate, while a host of challengers have criss-crossed the country campaigning against him.

The reason behind the low-profile approach ostensibly was to enable Reagan to keep collecting lucrative fees from his speech-making, radio commentary and ndwspaper column-writing activities. But the strategy also shielded him from press criticism and kept him from making any early blunders.

Reagan lieutenants, however, have been waging a full-scale campaign for more than six months. They've raised money, met with scores of political leaders, reactivated Reagan's 1976 campaign organization, studied issues and recruited an array of professionals. Reagan has lost some 1976 supporters in almost every state, but his campaign organization remains impressive.

The only serious weakness in the Reagan effort is fund raising. In late August, the campaign found itself almost $500,000 in debt, and Lyn Nofziger, a longtime Reagan aide, was removed as finance director.

In an interview, Nofziger attributed much of the fund-raising difficulties to the fact that Reagan had not announced his candidacy.But he also said that "corporate America, which is big government America, has decided that John Connally and not Ronald Reagan is its candidate." There is also a concern among conservative Republicans, he said, about whether Reagan "will remain a conservative" when he becomes a candidate.

This was a reference to efforts to "repackage" Reagan as a candidate with broader appeal than the conservative who nearly won the 1976 nomination. The repackaged Reagan is expected to formally become a candidate in mid-November.

Current plans call for him to open his campaign with a 30-minute national television address followed by a flurry of appearances around the country to make Reagan appear a vigorous candidate and waylay questions about his age. Reagan will be 69 in 1980.

The "new" Reagan will look and sound pretty much like the old Reagan, says campaign political director Charlie Black. But he will "speak with a lot more detail on a broader range of subjects" than the old Reagan.

"Instead of saying 'here's what is wrong with the system,' he'll be saying 'here's how I'll change it,'" Black adds.

Black and other Reagan spokesmen maintain Kennedy will help rather than hurt their candidate because Kennedy will draw attention to the Democratic race. This would make it hard for the public to focus on the Republican race, he says.

Black and political professionals in other Republican campaigns doubt that Ford will become a candidate. "I think he'd (Ford) like to be the nominee, but neither he nor his family are willing to do what it takes to become the nominee," he says.