WASHINGTON (AP) _ President Bush said Thursday night he would veto an attempt to raise the top marginal federal income tax rate to 33 percent on high incomes.

''I will veto a rate increase. You heard it for the first time here, but I have sent that signal up to the Hill,'' Bush said in a live interview on the ABC-TV show ''Prime Time Live.''

Bush was answering a question from Sam Donaldson about what he would do if Democrats on Capitol Hill succeed in passing their own tax bill instead of Bush's proposed cut in the capital gains tax. The Democrats are discussing the possibility of a competing proposal to restore the tax deduction for individual retirement accounts - IRAs - and raising the top rate to pay for it.

Because of a quirk in the tax law, the top rate is now 28 percent, but as income rises the rate jumps to 33 percent before dropping back to 28 percent. The Democratic plan would eliminate this so-called bubble and keep the top rate at 33 percent for all higher incomes.

Bush, who campaigned on the slogan ''Read my lips - no new taxes,'' defended his proposal for a cut in the capital gains rate, a variation of which has been cleared by the House Ways and Means Committee.

''It is not a tax break for the rich,'' Bush said.

Backers have said the lower top rate - 19.6 percent instead of the current 28 percent - would increase economic activity. But Congress' Joint Economic Committee has said 80 percent of the benefits would go to people with incomes above $100,000 a year.

On other subjects, Bush was asked whether he would meet Soviet President Mikhail Gorbachev in a summit conference.

''There will be one,'' he replied. But he declined to be pinned down on the time. Asked if it would be early next year, the president replied, ''I wouldn't say early.''

The interview was preceded by a live tour of the president's living quarters on the second floor of the White House.