WASHINGTON (AP) _ Republican leaders said Tuesday that House Speaker Thomas P. O'Neill hurt chances for reducing the deficit by using Social Security and Medicare issues in a campaign fund-raising letter, but a spokesman for O'Neill said the letter contained ''the truth.''

The letter, sent this year to potential Democratic contributors, predicted a GOP effort was imminent to ''slash Medicare benefits.''

''Unless you and I act immediately ... the future of Medicare will be dictated by Republicans intent on balancing record-high budget deficits on the backs of millions of older American citizens,'' said the letter sent out under O'Neill's name.

The letter was brought to President Reagan's attention Tuesday by House Minority Leader Robert Michel of Illinois.

The letter was ''just making it another political issue for the purpose of raising funds to 'save Medicare,' 'save Social Security,' and all the rest,'' Michel said after meeting with Reagan. ''You know, as long as that continues to go on, you're just not going to get the president to change or alter his position'' on the budget, he said.

Michel's father-in-law received the mailing, according to Michel's spokesman, Mike Johnson.

''He just got furious about it. He took it to the White House to show the president what he was up against,'' Johnson said.

The two-page letter, called an ''Express Dispatch,'' was mailed out this month and last month by the Democratic Congressional Campaign Committee. An aide said it was little changed from mailings during last year's presidential campaign.

President Reagan's fiscal 1986 budget does not call for freezing Social Security benefits although it does recommend a one-year freeze in Medicare payments to hospitals and some doctors and calls for increasing premiums for some Medicare recipients.

Senate Republican leaders, however, are supporting a one-year freeze in Social Security as part of an overall deficit-reduction package. The GOP congressional leadership met Tuesday with President Reagan on the budget.

After the meeting, White House deputy press secretary Larry Speakes said the president was ''exactly where he stood from day one. He does not favor a reduction in defense and does not favor a Social Security COLAs (freeze).

''He believes that if he or the Republicans propose a freeze in Social Security COLAs (cost of living adjustments), that the Democrats will take it up and use it to demagogue it in the strongest terms,'' Speakes said.

Senate Majority Leader Robert Dole called it ''a nasty letter.''

Christopher Matthews, a spokesman for O'Neill, said, ''They're upset by it because sometimes the truth hurts.''

He called the letter ''a bull's-eye'' and ''if it was inaccurate, they would have said it was inaccurate.''