A supposed Texas showdown between the two surviving Republican presidential candidates tonight turned into a mutual criticism session of President Carter.

In a polite, stand-up debate, Ronald Reagan and George Bush explored two longstanding points of disagreement, blockade of Cuba and the Kemp-Roth tax bill, but otherwise took most of an hour trying to match each other's jabs at the Carter Administration.

Reagan aides had expected Bush, fresh from his victory in the popularity contest portion of the Pennsylvania primary, to directly challenge Reagan's credibility on various issues.

Instead, Bush stood by without saying anything as moderator Howard K. Smith asked Reagan about printed and television accounts charging that he frequently mistakes the facts. One of these purported misstatements, said Smith, was a statement that the Department of Health, Welfare and Education spends $3 for each $1 of benefit it confers, when the actual figure is 12 cents spent.

"I wouldn't believe HEW if it was here in the room," Reagan said to laughter.

He then went on to defend various statistics he has used and to read a United Press International account which concluded that Reagan was more nearly correct than CBS in a dispute about the number of federal jobs which had been added under Carter.

Reagan used the figure 131,000, CBS said 6,000 and the UPI account said the correct figure was 63,000. But the story added that this does not include other jobs which the federal government is responsible for creating and added that Reagan may actually have understated the increase.

"I have confidence in the facts and figures I have used," Reagan said.

Bush's sharpest challenge was over Reagan's proposal to cut the federal income tax 10 percent a year for three years. Bush said this would push the present 18 percent inflation rate to 32 percent and further increase the federal deficit.

"I don't believe we can take that kind of risk, Governor," Bush said.

He proposed, as he has in the past, a $20 billion tax cut aimed at stimulating investments.

Reagan vigorously defended his proposal, saying that it would stimulate productivity to such a degrree that it would more than offset any loss of federal tax revenue.

"That money isn't going to be buried in a tin can in the backyard," Reagan said, "it's going to be used to buy things, and when it's used to buy things, productivity is going to increase, people are going to put it in savings accounts and we're going to have the capital to invest in research and development."

The other point of difference, also of long standing, was whether the United States should blockade Cuba as a response to the Soviets in Afghanistan.

"We have a fundamental difference, because it wasn't Cubans who invaded Afghanistan -- it was Russians," said Bush, arguing that it would be a mistake to link this hemisphere's problems with U.S-Soviet relations.

Reagan said it was the most effective way to get a response from the Russians.

"A little call on the hotline with this kind of threat might get some withdrawal from Afghanistan," Reagan responded.

Otherwise; the debate seemed to be a Republican unity speech on the evils of the Carter administration. Bush accused Carter of "reckless deficits," and Reagan agreed. Both candidates vied with one another in accusing Carter of weakness on Iran, and both disagreed with the suggestion of a Smith question that it would be dangerous for the United States to mine Iranian harbors.

Both Bush and Reagan said, also the United States should supply arms to the Afghans to resist the Russian invasion of their country.

Both Bush and Reagan agreed that Rep. John B. Anderson, who is expected to announce his independent candidacy Thursday, would take more votes from Carter than he would from the Republican nominee.

"I know I speak for George when I say we sure do miss him tonight," Reagan said, while Bush grinned broadly.

Near the end of the debate, after both candidates had refused to say who their vice presidential running mates would be, Reagan said that his running mate would have to support the Kemp-Roth tax bill.

Afterward, Reagan called this "an ill-chosen bit of humor" and said he hadn't meant to rule out -- or in -- Bush or anyone else. Bush called the debate "boring" and said, "it didn't have some of the zip the others had."