After nearly 20 years, Richard M. Nixon and George McGovern agree on something: Gov. Mario Cuomo of New York is the strongest presidential challenger the Democrats could run in 1992.

McGovern, loser to Nixon's 49-state presidential landslide in 1972, is thinking about running again himself, so he talked with Cuomo last month about the governor's plans, without discovering them."I don't know what he told me," McGovern said over breakfast the other day.

McGovern, 68, says he will decide within the month whether to enter a field so far left open by major Democratic officeholders. One by one, prime prospects for the 1992 nomination have been saying no - Sen. Sam Nunn of Georgia, Rep. Richard Gephardt of Missouri, Sen. George Mitchell of Maine. Cuomo is a maybe. So is Sen. Albert Gore Jr. of Tennessee and Sen. Lloyd M. Bentsen of Texas.

"I've said I was willing to take the risks involved if no one else, less cut up in the past, is willing to do it," McGovern said.

Another former senator, Paul E. Tsongas of Massachusetts, is running, and Virginia Gov. L. Douglas Wilder has a pre-campaign committee testing his prospects.

Those are not names that likely would dissuade McGovern. He has sent out a fund-raising letter to 3,000 potential contributors to see if he can line up the support in 20 states that would entitle him to federal campaign funds next year.

"If there isn't at least that much support out there, I ought to forget it," he said. And some of his friends and earlier supporters say he ought to forget it anyhow. "They say, `How can you be such a damn fool?' " McGovern said.

But he said there needs to be a liberal Democratic voice, soon, to fill what he considers a campaign vacuum. Party leaders say they aren't concerned about a delayed start in the challenge to President Bush. McGovern says they should be, because it will take time to build backing and hammer home issues.

Besides, for a man who earns his living lecturing, writing and teaching, the visibility of another presidential campaign could be its own reward.

McGovern said that if he does run and a liberal with a better chance of success enters the race later, "I wouldn't hesitate to step aside."

That description fits Cuomo. "He's a liberal, pretty much in my tradition," said McGovern - a political blessing that would be no asset in the conservative Sun Belt territory that any Democrat would have trouble cracking.

McGovern said Cuomo probably is the best orator, debater and television campaigner the Democrats have, with ready access to campaign funds and the New York governorship as a campaign base.

"I think that Cuomo's got the best shot at it," McGovern said. So, while convalescing after prostate surgery, he called Cuomo to talk about it. Cuomo, he said, talked instead about getting his state budget woes under control.

"I didn't hear the door slam," McGovern said.

Republican Nixon said that "Cuomo, without question" is the Democrat who could give Bush the toughest challenge next year. "The question is whether he can get New York state in good enough shape that he would be able to go from there and run nationally," Nixon said in a CNN interview last Monday. "But if he runs, he would give George Bush a good race . . . . "

Bush would win, Nixon said, although narrowly if the recession persists.

As for McGovern, Nixon said he might be foolish to run, "but look, who else is doing it?"

"He's never going to be nominated," the former president said, "but he might be able to play a role in seeing who is nominated."

McGovern, who ran briefly in 1968, upset the odds to capture the 1972 nomination, and waged another brief campaign in 1984, said he knows he would be the longest of long shots next year.

He ticks off his liabilities: He's been out of office 10 years since losing his South Dakota Senate seat, he managed to carry only one state against Nixon, and he's regarded as "a super liberal" in a conservative era.

"Those are lingering images that I'm going to have to figure out some way to deal with," McGovern said.

There's one more image that haunts him - Harold Stassen, whose name has become synonymous with perennial, futile candidacy. Now some of McGovern's friends are telling him he's risking the same reputation.

"It really bugs me," he said.