Jimmy Carter's embattled White House has become "the nation's fire hydrant," Vice President Walter Mondale said yesterday in an assessment of the troubled fortunes and complex burdens of office these days.

"To the American people, the thought that a politician is going to have a good night's sleep is an intolerable concept," Mondale observed in an expansive discussion on why he believes President Carter will pull ahead of Republican Ronald Reagan in the final weeks of the campaign. "And particularly when there's nothing at stake. They just love to keep us awake at night.

"I told Carter one day, I said I got this figured out. We're the nation's fire hydrant.

"But when you get right down to that last week or 10 days, the cold reality of who you really want to have as president of the United States really takes hold."

In contrast to the "mean" image that has fastened on the Carter campaign of late, the vice president exhibited his special brand of good-natured wit and ease over the brussel sprouts at a lunch with Washington Post editors and reporters.

While his message was often one of frustration, he smiled when he takled about the media's noncoverage of serious speeches, its failure to focus equally on the "amnesia" of Republican opponent Reagan and the general difficulties of being an elected official in 1980.

Much of the discussion centered on the feeling on all sides that the issues were being ignored or avoided -- by candidates and media alike -- too often in favor of cheap shots, or colorful but trivial controversies.

"I don't know how good this new Stealth technology is, but if you want to sneak into a town and be unnoticed, just try to give a positive speech," Mondale said, adding a wistful tally of American cities on which his earnest talks on issues had fallen into a media black hole.

"My basic function now is to fly around the country with national reporters in the hopes we get close to [some news.] I was in Arkansas when the Titan II went off, thank God. I was in Seattle when the boat was burning. I was in Massachusetts when Mrs. [Rose] Kennedy got sick.

"I've been suggesting we get in one of those long-range 747s and just circle and make a strategic strike wherever there's news."

When asked about outraged calls the newspaper has received from White House press secretary Jody Powell, complaining the press has not been fair, Mondale said, "You haven't had any calls from me. I learned the hard way -- you pay and pay and pay . . . ."

Urging a debate between the two principal candidates -- but not independent John Anderson -- Mondale said, "The American people know the candidates are giving the best possible case for themselves and . . . they are looking for an uncontrolled environment in which they might, just might, discover whether we've got the stuff or not.

"I've checked 16 years of news releases and I've never critcized myself once."

Mondale placed much of the blame for the Democrats' poor standing on Anderson's candidacy, but he said Anderson is fading and the process will accelerate.

At the same time, he added, "I'm not sanguine [about Anderson]. In 1976, [Eugene] McCarthy fell off the table and he still took four states from us.

"I'll tell you, I believe in the two-party system. I do not believe this floating crap game that some people like so much will do anything but weaken our country, weaken the presidency. . . ."

Mondale spent considerable time making a case for the Carter administration, defending the administration record on the economy, foreign affairs, energy and, last but not least, its major contribution to upgrading the office of vice president. He said a major plus for an incumbent is peoples' understanding that being president is the only place to get good experience in how to be president.

One of the many things this president has learned, he said, is "the crucial nature of the public education role of the president" -- his ability to set the agenda for public discussion. The lack of this particular knowledge "may be the biggest inadequacy of the first part of our administration."

"That's the main power. If you asked me, if I had to give up one -- the opportunity to get on the evening news, or the veto power, I think I'd throw the veto power away. It's the president's most indispensible power."