Republican National Chairman Bill Brock said yesterday that President Carter's proposed $25 billion tax cut was a "highly political" measure timed to take effect five weeks before election day, but that next year Americans will be paying more taxes than they are now.

"He failed to mention the timing [of the cut], that will take effect Oct. 1," Brock said in a news conference at the Republican National Committee's winter meeting. Because of increased Social Security levies and adjusted income tax withholding schedules, taxpayers on Jan. 1 will face increases in levies, he said.

"Most people will be fortunate to break even," Brock added in an informal critique of the President's State of the Union message.The Republican Party later will issue its formal response to the message.

Brock said the party will press for a 30 per cent across-the-board tax cut over the next three years to stimulate the economy. Such a reduction, he said, would not necessarily increase the rate of inflation, and would surely reduce joblessness.

Carter's message Thursday night, Brock said, "offered stark contrast to the promises he made recently in the 1976 campaign.

"The theme sounded almost apologetic," Brock said, adding, "I do agree that the state of the union is sound, but the state of the government is not."

He said that in spite of Carter's litany of the administration's accomplishments, the President "has nto effectively led Congress to deal with our problems . . . There is no substance to the proposal to deal with those problems."

Brock said he had met with some of the striking farmers who are in Washington seeking price support increases and said he had sympathy for them because the administration's response has been "another case of Carter's broken promises.

"It looks as if the administration is going the wrong way, putting more controls on them instead of expanding their markets," he said.

He called Carter's jobs programs "inadequate," and said that while the President emphasized productivity in his energy program, there were no specific proposals to accomplish increased productivity.