The two top congressional Republicans, seeking to dispel the pessimism clouding GOP chances in next month's general election, predicted yesterday that Republicans would lose only 10 to 12 House seats in scattered races nationwide.

Despite the recession and continued high unemployment, Senate Majority Leader Howard H. Baker Jr. (Tenn.) said, he does not expect that any GOP Senate incumbents will be defeated. Depending on what he called the "breaks of the campaign," Republicans could increase their total of 54 seats in the Senate by one or two, Baker said.

House Minority Leader Robert H. Michel (Ill.) said that, while there may be problems holding together the "fragile coalition" of House Republicans and conservative Democrats who have voted for President Reagan's programs, he does not believe Republicans will lose more than 10 to 12 of their 192 seats.

"We find through our polling and through the candidates themselves we're really in much better shape than what we would have dreamed of earlier this year," Michel said.

Baker and Michel discussed their optimistic outlook after a meeting with Reagan. Their upbeat predictions contrast with private forecasts of presidential advisers and exceed assessments considered realistic by many GOP strategists.

While few believe that Republicans will lose control of the Senate, pessimistic GOP strategists have begun telling reporters privately that they fear Republicans could lose as many as 40 House seats. News stories reporting these views have clearly caused consternation among Republicans on Capitol Hill.

"I suspect that what the White House is doing is trying to build up expectations of a huge loss so it doesn't look like a huge loss," Rep. Guy Vander Jagt (R-Mich.) told Associated Press yesterday. "What they're trying to do is to paint a dismal picture so that, no matter what we do, it will look like a victory.

"I don't like to play psychological games like that."

The party in power traditionally loses seats in Congress in the first off-year elections after winning the White House. Over the last 60 years, such losses have averaged about 35 seats. Since 1954, however, those losses have averaged only about 12 seats.

Michel, expressing concern about the potential collapse of Reagan's ideological majority in the House, forecast two years of "confrontation and stalemate" between the House and White House if that were to happen. But he said he thinks the GOP will lose seats only in "isolated instances, in pockets of high unemployment."

Baker said he believes that the economy "cuts both ways" as a campaign issue. While joblessness is up, he said, people are grateful that the Republicans have been able to reduce interest rates and stem the rise in inflation.

Vice President Bush made similar claims yesterday in a campaign swing through Michigan, which has the nation's highest level of unemployment. Bush said he is confident that the public will remain patient as Reagan deals with economic problems because they realize "Rome wasn't built in a day."

Democratic Sen. Paul E. Tsongas (Mass.) said in a speech at the National Press Club yesterday that "anti-Reaganism is the core of the Democratic appeal" in this fall's elections and all-but-certain Democratic gains.

Tsongas, regarded as a leader of neoliberal Democrats, said his party must devise new alternatives to Reagan's policies if Democrats expect to regain the White House in 1984.

Meanwhile yesterday, House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), stressing the jobs theme of the fall campaign, promised to give "top priority" after the election to public service jobs programs that Reagan opposes. O'Neill's emphasis on his philosophical differences with Reagan came in an unusual ceremo-ny in which he signed a jobs training bill Reagan favored.