With the publishers divided, newspaper unions struggled yesterday to come up with a united facade in the 53-day strike that has closed this city's three dailies.

New York Post publisher Rupert Murdoch chipped a new hole in publisher's unity when he reached a tentative agreement yesterday morning for a new contract.

Union president Douglas LaChance said his drivers would not cross other unions' picket lines at The Post, but the tentative agreement clearly was intended to threaten The New York Times and Daily News with the possibility that Murdoch can settle individually with the others.

All three publishers have automated equipment and enough nonunion personnel to be able to publish if the delivery drivers would cross the picket lines. But LaChance said:

"We have no intention of crossing pressmen's picket lines at the Post or anywhere else."

Murdoch has established himself as the joker in the deck. Asked what The Post's publisher was up to, federal don't know. He's got more things going than Carter's little liver pills."

At a press conference after a meeting of all the unions, their adviser, labor lawyer Theodore Kheel, accused Murdoch of having "made up out of whole cloth" a report that a secret arrangement was being worked out with the connivance of the Times, Daily News, Kheel and the other unions to be jammed down the striking pressmen's throats.

In anger and suspicion that a deal was being cut behind his back, pressmen's union leader William Kennedy walked out of the negotiating session yesterday.

Kheel was asked what motive Murdoch might have for disrupting the talks and thereby prolonging the strike.

"I have a suspicion," Kheel replied. He said he had heard a rumor that Murdoch wants to stretch out the strike in hopes that the financially delicate Daily News will be crippled and Murdoch could invade the morning market here. His afternoon Post is losing money fast when it is publishing, and his other two properties here, the weekly Village Voice and New York Magazine, are flourishing on extra advertising and circulation while the three dailies are struck.

Kheel said he found the rumor incredible when he first heard it but was no longer sure.

While Kheel and Murdoch escalated the verbal duel that began when the unions brought the adviser into the talks almost three weeks ago, pressman Kennedy was pleased with the split among the publishers. He said he always believed negotiations would progress more quickly if he could meet each publisher separately.

Kennedy told reporters after a lengthy session with Kheel and leaders of the other unions that he now was satisfied that no one was negotiating behind his back and that the other unions support him. He said he would re-enter talks with The Times and News today and hopes to meet separately with The Post next week.