Thirty-one congressional Democrats who provided crucial votes favoring last year's humanitarian aid to Nicaraguan rebels urged President Reagan yesterday to postpone his request for more aid until a final regional peace effort can be made.

At the same time, Rep. David R. Obey (D-Wis.), head of the House Appropriations subcommittee on foreign affairs, warned that new balanced-budget legislation means that a 25 percent reduction in foreign aid programs, including aid to the rebels, will be "the best the administration stands to get out of my committee . . . and it will probably be more like 50 percent."

In a letter, the Democrats asked for "a major diplomatic initiative" supporting the Contadora effort by Mexico, Venezuela, Colombia and Panama to negotiate a Central American peace treaty. Foreign ministers of those countries are to meet here next week with Reagan and foreign ministers of Peru, Brazil, Argentina and Uruguay, which comprise the Contadora Support Group.

Heads of state of the Contadora nations and Costa Rica, Honduras, Nicaragua and El Salvador accepted an invitation from newly inaugurated Guatemalan President Vinicio Cerezo to meet there next month.

Latin diplomats have called these gatherings Contadora's last-ditch effort to unsnarl talks stalled since last autumn.

Twenty-five of the 31 Democrats were among 68 members who opposed military aid to the counterrevolutionaries, called contras, but later voted in favor of Reagan's $27 million humanitarian-aid package.

That aid expires March 31, and administration officials have said the president will seek at least $100 million in renewed humanitarian and military aid when Congress returns Feb. 18 from its holiday recess.

"It would be counterproductive to vote on the aid then," said Rep. James C. Slattery (D-Kan.), who wrote the letter. "A defeat would send the wrong message to [Nicaraguan president] Daniel Ortega, and a win would send the wrong message to the Contadora group" before its meeting.

The letter asked that the contra aid request be delayed until mid-March, that Reagan agree to meet with the Contadora heads of state if they request it, and that he create a bipartisan delegation of members of Congress and administration officials "to explore how the United States can best support Contadora."

Rep. Dave McCurdy (D-Okla.), another signer and a member of the House intelligence committee, said that no aid package would pass "until the president can make the case that he has pursued all diplomatic alternatives."

Rep. Richard A. Gephardt (D-Mo.) said he thinks that the signers of the letter "will vote later on that aid request, depending on how the president has responded to us."

Obey said in an interview that he sees "no chance for any supplemental foreign aid request whatsoever" for fiscal 1986. Senate Foreign Relations Committee Chairman Richard G. Lugar (R-Ind.) said as much last week.

Levels for 1987, including contra aid, will be at least 25 percent below this year's appropriations and, with the exception of Israel, Egypt and Pakistan, could be "more like 50 percent," Obey said.

"I led the opposition" to the Gramm-Rudman-Hollings balanced-budget act, Obey said. "But, if the administration is not willing to lead the way out of this irrational box that they have put the country in, they had better expect me to be the toughest enforcer of [it]."