NEW YORK, SEPT. 14 -- The New York Post, the snapping, snarling tabloid that mirrors the rambunctious nature of city life, staggered past its biggest deadline tonight as union leaders reached tentative agreement on $20 million in cutbacks that would save America's oldest continuously published daily newspaper from extinction.

But the Post's largest union, the Newspaper Guild, which represents reporters and editors, still must approve the pact in a vote scheduled for Monday. Ten other unions at the paper reached final agreement, including substantial layoffs, with owner Peter Kalikow after a tension-packed day of talks that was capped by last-minute mediation by Mayor David N. Dinkins (D).

The agreement still could unravel if the Guild's 350 members, many of whom are opposed to wage cuts or layoffs, reject the proposal. "If they turn it down, the paper will stop publishing immediately," Kalikow said at a news conference just before midnight.

Barry Lipton, the Guild president, said he would recommend the agreement. "It's the very best we could do," he said. "We've gone as far as we can. Mr. Kalikow has gone as far as he can."

Kalikow said he would arrange for employees to buy some of the newspaper's stock and hold a seat on the board of directors if the fragile agreement holds up.

The marathon talks were so delicate that the Post itself prepared two stories for Saturday's editions -- one announcing the tabloid's obituary, the other its rescue -- and held them until the last possible moment.

Both sides were tight-lipped about the proposal to the Guild, which earlier rejected Kalikow's demand for a 46 percent wage and benefit cut or four-day work week. It was not clear whether the Guild had accepted an alternative of roughly 100 layoffs, which some staffers said would decimate an already meager news staff and amount to placing the paper on life supports.

Labor spokesman George McDonald said "we feel like crying" over jobs eliminated in the craft unions but that "we have to save what jobs we can. Everyone's getting clobbered here."

As if to underscore the need for a miracle, negotiators earlier in the day gathered at the midtown residence of Cardinal John J. O'Connor, who prayed for the paper's survival.

The Post, founded by Alexander Hamilton in 1801, later mutated from a staid liberal voice under Dorothy Schiff in the 1950s and '60s to a screaming, Fleet Street-style rag under Rupert Murdoch in the 1970s and '80s to a saucy hybrid, with the city's most conservative editorial page, under Kalikow.

"We had the Post in my house every single day when I was a kid," said reporter Esther Pessin, who has chronicled the exploits of Donald Trump, Leona Helmsley and other bigshots. "It just won't be New York without the Post. It just won't be the same."

"We're a rock-and-roll newspaper," reporter Bill Hoffmann said. "You could write some piece of garbage . . . puppies saved from being squashed on Fifth Avenue . . . and the next day it'd be the lead story on television and radio because the Post put it on Page One."

Kalikow, a Manhattan real estate developer who bought the paper from Murdoch in 1988 for $37 million, has been under pressure from his bankers to stem losses he estimates at $27 million a year.

Reporters and editors have not had a raise in three years. The drivers' union had rejected a deal that would have axed more than 100 of its 263 jobs.

There was some financial incentive for the paper's 850 unionized employees to force a shutdown now -- when Kalikow is required to give them 60 days' salary, severance and vacation pay -- rather than waiting until contracts expire next March.

If the Post sometimes has seemed obsessed with Trump's love life or Elizabeth Taylor's latest diet, it also has produced bursts of serious journalism. These have included stories on widespread cheating on state Regents exams ("EASY AS PI") and on sexual and financial improprieties by the Rev. Bruce Ritter that forced his resignation from the Covenant House shelter for runaways.

But Kalikow lost $25 million on a Sunday edition that lasted just three months and watched advertising revenue plummet in the 500,000- circulation daily as the regional economy turned sour. Critics noted that Kalikow delivered his ultimatum shortly after winning city approval to demolish several Upper East Side apartment buildings that had been declared landmarks.

With the New York market unable to support four dailies, the Post has long been in danger of joining the Washington Star, Philadelphia Bulletin, Chicago Daily News and Los Angeles Herald-Examiner in the graveyard of once-proud afternoon papers done in by television, traffic congestion and readers fleeing to the suburbs.

The Daily News, owned by the Chicago Tribune Co., has been locked in a bitter struggle with the same unions over demands to reduce staffing levels and has served notice that it would seek any concessions granted the Post under a "me-too" contract clause.

Along with strong sports coverage and columnists from Pete Hamill to Ed Koch, The Post is known for headlines like "UP YOURS!" (to Saddam Hussein) and "BARR-F!" (after Roseanne Barr mangled the Star-Spangled Banner).

"If you had a particularly catchy head on a big story, you might sell 20,000 more papers," said V.A. "Vinnie" Musetto, who wrote some of the best as front-page editor in the mid-1980s. "My own favorite was 'GRANNY EXECUTED IN HER PINK PAJAMAS,' about a Florida woman going to the electric chair. Then there was 'TERROR FROM THE SKIES.' I almost got fired over that one." It was about a falling air-conditioner.

Just before noon today, a cab driver stopped in front of the paper's South Street building and shouted at Editor Jerry Nachman, "Hey, Jerry! Any progress in the talks?"

Nachman lit a cigarette and reflected on "the remarkable coverage we provided in the last year with a cityside staff smaller than the New York Times's Washington bureau. The most interesting thing in my life is to uncover on a daily basis how little people who only read the New York Times know about New York."

Nachman was joined by Timothy McDarrah, a "Page Six" gossip reporter wearing a tuxedo for the occasion. "Granted, we don't have anybody in Kuwait, but when high school students have a black market in exams, we're all over the story," he said. "We get it first. We set the agenda."

McDarrah's reflections on his craft could have served as the Post's epitaph: "We shoot straight and play fair. We may say you're sleeping with your neighbor's wife, but at least we'll call you first and get a denial. We're not scared of nobody. We'll ask anybody anything at any time. We're the mouth of the public."