When he arrived on the House floor yesterday for the roll call on President Reagan's veto of the $14.1 billion supplemental appropriations bill, Rep. Clay Shaw (R-Fla.) was torn.

Elected in the Reagan sweep of 1980, Shaw has voted repeatedly with the president in the budget-cutting battles of the last 20 months. He backed the austere Republican budget resolution in June, and in July voted to sustain Reagan's veto of a "budget-busting" appropriations bill.

But this time, Shaw was also hearing from the senior citizens in his south Florida district who feared that the Reagan veto, if upheld, would wipe out a popular community services program for the elderly. And he worried that it would also wipe out Reagan's own $350 million Caribbean Basin Initiative, a matter of some consequence to south Florida's refugee-flooded economy.

Shaw voted for the elderly and against Reagan yesterday. He was one of a number of fiscal conservatives who have been Reagan soulmates in the past but deserted him yesterday.

They contributed to the 301-to-117 vote by which the House handed Reagan a unexpected defeat and voted to override his veto of the spending measure eight weeks before Election Day. It was a setback for Reagan, at the hands not only of his Democratic opposition but also of his Republican allies who didn't think this money-saving veto was worth the fight.

House Minority Leader Rep. Robert H. Michel (R-Ill.) acknowledged as much after the vote, saying Reagan's veto was "not a good, solid position to defend." Although Reagan had labeled the bill a "budget-buster," Michel said many Republicans didn't see it that way because the total was nearly $2 billion less than the president had requested, only the specifics were different.

There were only "tradeoffs" to be made, Michel said, between social programs and defense. And this time, Reagan's allies decided they didn't like the looks of the tradeoffs he wanted.

There will be other such votes in the weeks ahead. The closer the election comes, the harder it is likely to be for members to choose between the president and the spending programs he wants to cut.

Shaw, for example, worried about the $211 million item in the bill for a community services program that employs 54,000 older Americans. Michel tried to reassure House members that this popular program would be funded in a later appropriations bill, but Shaw and others had doubts.

"I initially supported that legislation," said Shaw, the former mayor of Fort Lauderdale. "This is an important bill in Broward County."

Although only about 300 elderly people in his district work directly for the program, Shaw said, it "literally touches" thousands of older citizens across Florida, where the elderly carry sizable political clout. Eleven of the 15-member Florida delegation voted against Reagan yesterday, including Shaw and Rep. Bill McCollum (R-Fla.), both of whom have backed the president in previous budget fights.

In recent weeks, senior citizens groups had alerted their members to the possibility that the program would close down Sept. 30 if Reagan's veto of the "regular" supplemental appropriations bill was upheld. And that possibility "very definitely affected" the Florida delegation's voting, Shaw said.

Even though White House lobbyists urged him to back Reagan again, Shaw said the pleas ran against his own feeling that "it's tough to come back and tell a member to change position in an election year. That can be a good issue for your opponent."

McCollum said he decided to vote to override the veto because he didn't think the appropriations bill was a "budget-buster" as Reagan claimed.

"I wasn't busting a thing," he said. "I hope the president will veto a lot of appropriations bills," if they do exceed the budget, he added.

Other Reagan allies found other reasons to vote against the president. Rep. Thomas B. Evans Jr. (R-Del.) said the supplemental bill contained needed money for the Coast Guard, which Congress has given new responsibilities. There were "at least 50 or 60" members "who feel as I do that we want to reduce the defcit but there are better alternatives for doing it," Evans said.

In addition to the defections among Republicans -- 81 of whom voted to override the veto and 104 to sustain it -- Reagan also lost the support of his Democratic budget-cutting allies, the Boll Weevils. Of these 47 conservative Democrats, 34 voted to override the president's veto.