Congress probably will vote today to sustain President Reagan's veto of a $14.1 billion appropriations bill but Reagan will still have to accept less for defense and more for social programs than he wants, House Republican leaders said yesterday.

Their assessment came as the administration put out feelers on Capitol Hill for a post-showdown compromise that would contain $300 million of the nearly $1 billion that Congress wants for social welfare programs, along with $350 million for economic aid to Caribbean Basin countries and more money for defense.

House Minority Leader Robert H. Michel (R-Ill.) warned, however, that Reagan may have to back down on military spending and said the Caribbean aid money is in doubt. He also said Reagan will probably have to accept some funds for jobs for the elderly and added:

"It's unrealistic to think the president is going to get everything he wants without the Congress getting a nickel of what it wants."

While Democratic leaders privately conceded Reagan has a strong edge in votes, largely because a two-thirds vote of both houses is required to override a presidential veto, they continued trying to match him word for word in the propoganda battle over what they derisively described as "this sideshow."

"The sad fact is that the president's veto of the supplemental appropriations bill is based not on any economic logic, but on politics and public relations," said House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) in a "Dear Colleague" letter that was also signed by Majority Leader James C. Wright (D-Tex.) and Whip Thomas S. Foley (D-Wash.). "The politics of the veto are all too obvious: It distracts attention from Reaganomics," the three Democrats said.

This letter followed a statement from the White House appealing for support for sustaining the veto. "Tomorrow's vote in the House of Representatives on my veto will be another important signal to the American people of whether Washington has the will to discipline government spending," Reagan said.

Nearly half of the $14.1 billion measure would go to meet federal payrolls and is essential to keep most of the government, including the Pentagon, operating after the middle of next week. The administration objects to nearly $1 billion of the rest, contending that it exceeds what the president wants to spend on social programs, especially for students and for the elderly. It also complains that Congress provided only $500 million of the $2.6 billion that it wanted for defense.

But, because the vetoed measure includes money for the president's high-priority Caribbean Basin iniative, Congress has some leverage to get the administration to compromise on domestic spending.

Even with a compromise on social programs, however, the Caribbean money is in doubt, some Republicans said. "The administration would certainly like it in the bill , but the political winds suggest it's out," said a Republican leadership aide.

Some of the strongest support for the bill stems from its inclusion of $210.6 million for jobs for elderly people. Rep. Silvio O. Conte (R-Mass.), ranking Republican on the House Appropriations Committee, was one of several House members to join with senior citizens groups yesterday to appeal for override of the veto in order to guarantee the jobs money. Also, 115 House members signed a petition calling for an override to save the jobs program for the elderly. "There's no doubt about the administration's intent," said Conte. "They want to do away with" the program.