Congress likely will cut personal and corporate income taxes by about $20 billion next year, Rep. Barber Conable of New York, the ranking Republican on the House Ways and Means Committee, predicted yesterday.

Conable told reporters he expects such an election-year cut even though it probably would make a balanced budget impossible to achieve in 1981.

His assessment differs considerably from that of the Way and Means chairman, Rep. Al Ullman (D-Ore.), who on Monday all but ruled out such a tax cut because of the need to balance the budget to fight inflation.

Conable also said he expects President Carter to ask next year that a Social Security tax rate increase scheduled to occur on Jan. 1, 1981, be reduced or eliminated because of its impact on poorer workers.

The New York congressman ticked off three reasons why he expects Congress to opt for a tax cut instead of balancing the budget:

"It's an election year."

"We've had a high rate of inflation" and, because that tends to push individuals into higher tax brackets, "we will try to keep the tax rate from going up too much."

"We've created an expectation of a tax cut every couple of years."

Conable further predicted that the cut would follow the lines of other recent reductions with about one-third going to business and about two-thirds to individuals.

Because of last year's reduction in capital gains taxes, he said he doubts that any other cuts intended to spur capital formation will be adopted soon. Therefore, along with Rep. James Jones (D.Okla.), Conable intends to propose changes in the way business accounts for depreciation on its equipment to allow a faster recovery of capital.

Ullman and Sen. Russell Long chairman of the Senate Finance Committee, had told a group attending the U.S. Chamber of Commerce annual meeting that balancing the budget should come first. "The need for balancing the budget eliminates any possibility of a major tax reduction in fiscal 1981," Ullman said.

Conable is convinced, however, that election-year imperatives will push the two chairman, along with the rest of Congress, to a more standard response to constituent pressure-cutting taxes. CAPTION: Picture, REP. BARBER CONABLE . . . a $20 billion tax cut in '80