WASHINGTON (AP) _ Senate opponents of a constitutional amendment to require a balanced federal budget claimed they had the votes to kill the proposal today while supporters said the outcome was too close to call.

''I believe we have the 34 votes we need to defeat it,'' Sen. Alan Cranston, D-Calif., the opponents' vote counter, said today.

The proposal to require Congress to appropriate no more money each year than the government collects in revenues must pass the Senate by a two-thirds margin, or 67 votes. Cranston said he may have even more than the 34 votes needed to defeat the amendment.

But Randy Rader, an aide to Senate Republicans, said Cranston's prediction may be overly optimistic. ''He may have 34 and we may have 67,'' Rader said.

''We won't know for sure until six o'clock,'' when the Senate is scheduled to vote, he added.

''We're doing all we can to try to get every vote,'' Sen. Strom Thurmond, R-S.C., one of the co-sponsors, said Monday night.

''It's going to be close,'' he added.

Thurmond and three other senators pushing the measure met Monday with President Reagan to discuss strategy before the vote.

The proposed amendment would also have to be approved by a two-thirds majority in the House and ratified by 38 states.

''We're within a couple, three votes'' Majority Leader Bob Dole, R-Kan., told reporters after leaving the White House meeting with Reagan.

But Dole said earlier Monday that he would prefer to postpone the vote until after Congress takes its Easter recess. A two-week delay would give supporters more time to line up votes. The outcome of an effort to shut off a filiguster against an unrelated bill was not expected to affect the scheduled action on the proposed amendment.

''It's going to come down to one or two votes, one way or the other,'' Sen. Orrin G. Hatch, R-Utah, said after the White House meeting.

Added Paul Simon, D-Ill., another meeting participant, ''Whether you're a conservative or a liberal, it does not make sense to spend an increasing percentage of your tax dollar on interest rather than goods and services.''

The amendment requires that any deficit spending measures or bills to raise the national debt limit be passed by a roll-call vote of three-fifths of each chamber. Tax increases would have to be approved by a bare majority of both houses - 51 senators and 218 House members.

Opponents led by Sen. Daniel J. Evans, R-Wash., contend that the amendment would not result in a balanced budget but would spawn extensive court battles over the spending authority of Congress.

A private consultant's study released Monday by Evans and Sen. Howard Metzenbaum, D-Ohio, concluded that the amendment would jeopardize federal savings deposit insurance as well as government-guaranteed loans to farmers and students.

''If the debt limit were reached or if outlays were to exceed receipts for a given fiscal year,'' the Federal Deposit Insurance Corp. would not be able to pay depositors of failed institutions, said the study, conducted by Wendell Belew, former chief counsel of the House Budget Committee.

''Payments to depositors could not be made ... unless there were sufficient income generated from insurance premiums and interest revenues to offset them,'' the study said.

''Under the proposed constitutional amendment, depositors in federally- insure d institutions would have to wait until a super-majority of both houses and the president resolve their differences before receiving their money,'' it said.

Fifty House Democrats led by Rep. Charles Stenholm of Texas, meanwhile, urged their Senate colleages in a letter released Monday to vote for the proposed amendment.

''At a time when continuing $200 billion deficits threaten the economy and the prospects of achieving even modest deficit reductions in FY 1987 remain uncertain, such a vote would send the nation a critically important message about the commitment of Congress, and of our party, to put the government on a sound, fiscally responsible footing,'' the House Democrats said.