Accusing President Carter of breaking his word on defense, inflation and unemployment, Republican presidential hopeful Ronald Reagan said today that America "cannot afford four more years" of Carter's leadership.

Speaking from the steps of the Ohio State Capitol, while Carter was addressing a rally nearby, the prospective GOP nominee underlined his differences with his general election opponent on both domestic and defense issues.

Reagan's speech was cheered by a crowd of about 5,000 to 6,000, which fell far short of filling the lawn of the capitol. It was slightly smaller than Carter's audience.

In a dress rehearsal for the coming fall campaign, Reagan set out to demonstrate -- as he told an audience in Dayton just before coming here -- that while he and Carter were close physically this noon, "I think spiritually, mentally and philosophically, we're a million miles apart."

Reagan also traded long-distance insults with Carter over who was the more conscientious governor. In an interview with newspaper editors on Wednesday, Carter said he recalled Reagan as "one who would come into a meeting [of the governors association] without doing the long, tedious work in which most of us were involved, would call a press conference and because of his fame, would attract a great deal of press attention and then he would be gone." But Carter said he did not dislike Reagan.

"I think he's describing Jerry Brown," Reagan said today in Ohio, referring to his successor as governor of California. "I remember staying from the first to the last session. And I remember a young newcomer coming aboard as governor of Georgia. For the life of me, I can't remember anything he ever did."

On issues, Reagan said in his speech that rather than adding taxes to discourage energy consumption, as Carter has proposed, he believed "we can have energy abundance if we turn the energy industry loose and get government out of the way."

Rather than accepting higher unemployment as the price of curbing inflation, as he said Carter is doing, Reagan said he favored an across-the-board tax cut for everyone. "It's not our buying things that causes inflation," he said. "It's government spending more than it takes in."

Rather than proposing resumption of draft registration, as Carter has done, Reagan said the way to "start the restoration of the strength of America is by having a realistic pay scale that induces young men and women to make the military their career."

He accused Carter of hypocrisy on the military pay issue. Reagan said that Carter had promised sailors aboard the carrier Nimitz on Monday to support a boost in their pay, but had told a liberal group at the White House two days later that the congressional budget resolution provided too much money for defense and too little for domestic programs.

"Will the real Mr. Carter stand up?" Reagan demanded.

While much of his speech was the standard rhetoric of his primary election campaign, the jibes at Carter were sharper than usual. They indicated that Reagan was trying out lines for the general election campaign, now that the withdrawal of George Bush has ended the last challenge to his nomination.

He ridiculed Carter's claim that the fiscal 1981 budget will be balanced, saying that Carter is "over-estimating revenues and underestimating expenditures," once again. And he ripped into other aspects of the administration's economic record.

"The president who said he'd never use unemployment to fight inflation is now saying we have to raise unemployment 1 1/2 percent this year to curb inflation," Reagan said. He called Carter's economics "old-fashioned and ridiculous." Noting that 825,000 people were added to the unemployment rolls last month, he said, "we need more jobs, not less."

"When he took office, inflation was 4.8 percent," Reagan said, "and he [Carter] said he'd do something about it. And he did. The rate is 16.4 percent for the first four months of this year.

"Gasoline was 58 cents a gallon, and he said he'd do something about it. He did -- and now it's $1.20."

"The government has to be turned around," Reagan declared. "We can't afford four more years of what we've had."