We are already an hour late for the rally in White Plains, and the big Secret Service limousine is still battling impossible traffic on the Cross-Bronx Expressway, but in the back seat Edward M. Kenndy is calmly contemplating a more abstract political problem: Where do you draw the line between gallantry and foolhardiness?

Without a victory of surprising proportion here Tuesday, Kennedy's chances of winning a majority of delegates to the Democratic National Convention will be close to hopeless -- and he knows it.

But all last week he talked about staying in the presidential race anyway to maintain liberal pressure on President Carter. And in a long, casual conversation, en route to White Plains, Kennedy sounds as if he means it.

"The heart of what I've talked about is, you know, treating people fairly," he says, sucking slowly on a long cigar. "For me, that's what the Democratic Party has been and ought to be. And, I mean, I honestly think you can get that idea across . . .

"One theory [among Kennedy's advisers] is what I ought to go back to the Senate and use it as a forum . . . but it doesn't ring right to me. My own sense is that you make a point, I mean, you know, if you come personally as a candidate and tell people what you believe in in a way you can't on the Senate floor."

To some, the role Kennedy seems to be designing for himself will sound attractive -- the gallant liberal pushing his cause after his personal hopes have been destroyed. But there is another, less charitable view -- the last of the Kennedys reduced to the status of a pesky gnat, a foolish spoiler who won't admit the obvious.

"Yeah, I hear you," Kennedy said. "There are -- some people are going to say that, yeah. But I don't know. If you have a person who really feels the issues, you're going to get a response.

"I really think I can take any hall in the country and talk about [national] health care and I really feel that one, and you can get people committed to it, they feel it, and nobody's going to say you're silly . . . You can hang in."

To Kennedy, it must be noted, this whole line of questioning is hypothetical, because like any candidate this one is still talking about winning.

He will "do very well" in New York and Connecticut Tuesday, he says, and go on to Wisconsin and Pennsylvania, which ought to be receptive to his liberal ideas, and eventually Jimmy Carter's popularity will come tumbling down.

"When this thing collapses for Carter, it just all falls apart in a few days, a week or two, you know," he says. "It's coming, you know it's coming -- there's no question about it."

This has been the official line in the Kennedy campaign for months now and the candidate has been saying it (somewhat less directly) everywhere he goes. But when the TV lights switch off and the microphones recede, Kennedy will acknowledge that his own campaign has problems.

"There's something to be said, you know, for going back to the Senate, for that idea," he observes in a speculative tone. "Among other things, that breaks up this pack [of reporters covering Kennedy] and maybe you guys can move off my family then and focus on Jimmy."

Kennedy admits as well that he has a lot less campaign money than he needs and will probably be in worse shape if he doesn't pull up somewhere near Carter in the delegate totals after this week's primaries.

And he knows, accordingly, that next Wednesday and Thursday, when he will be in Washington to assess the future of his campaign, there will probably be friends urging him to pull out of the race. At the moment, though, he seems to be girding himself to answer their arguments.

The money problem? "You know when you have this business of three or four, five little events here and there in a day, and you need advance people and you rent rooms, and all that, I mean, you run into big expenses.

"But I can get, you know, one smart advance person. I can go to two colleges, a union hall, in a day, and I can do that on a pretty thin budget."

Further, Kennedy says, as a Kennedy he is almost guaranteed television coverage wherever he goes, which will make up, to a small degree, for a depleted advertising budget.

The risk of splitting the party? "The risk, you know, is really, I mean, my worry is what's going to happen to the Democratic Party. They're already moving over to [Ronald] Reagan, that's what those budget cuts are about, and like I'm saying the rich can invest their money at 19 percent and fly by, but Jimmy's going to fight inflation with the poor. That's the risk to the Democratic Party."

The risk of looking silly? You know Mo [Rep. Morris K. Udall (D-Ariz.)] went to the convention and it was clear he hadn't gotten the delegates. I think Mo did a great job. And I think people now respect him for what he did."

But here, of course, there is a noticeable difference. Udall in 1976 may have been perceived, as Kennedy apparantely is today, as a big-spending liberal but he had more of the personal difficulties, none of the distrust and outright hatred that Kennedy faces as a result of Chappaquiddick and his other personal problems.

Does that mean that Kennedy might not be the best advocate for the liberal cause? This is the question that Kennedy does not want to deal with. lHe bristles, twirls the cigar, looks out the window into the night.

"How long before we get to White Plains, anyway?"