In separate radio addresses keyed to the Labor Day holiday and the traditional start of the campaign season, President Reagan and former vice president Mondale squared off yesterday on the subject of unemployment and which political party is responsible for it.

Speaking from the patio of his mountaintop ranch in California, Reagan tried to share the blame for the nation's 9.8 percent unemployment rate with his predecessors, saying that "unemployment has been gaining on us for years."

After observing that the unemployment rate since the mid-1970s has been far higher than in earlier postwar years, Reagan declared: "I guess what I'm trying to point out is that our unemployment problem is due to more than just the present recession. We must not only work our way out of the recession; we must adopt policies that will stimulate economic growth and create new jobs for the increased numbers entering the job market."

Mondale said Reagan's policies have divided the nation into "two Americas" -- one for the rich, the other for the poor.

"In Reagan's America, there's a long list of people waiting to buy Rolls Royces while a quarter of a million auto workers are waiting in line to find jobs," Mondale said in the first of a series of radio talks designed to boost the Democrats' 1982 congressional campaigns.

Reagan's five-minute speech was devoted largely to a defense of his economic policies, including his veto of a $14.1 billion supplemental spending bill that he said "would drive up spending once again."

After the speech, White House spokesman Larry Speakes blamed Congress for pending layoffs in the Internal Revenue Service and other agencies. Speakes said the employes who will be laid off are "innocent victims of congressional overspending." Mondale, in turn, accused the administration of trying to "finance its tax cuts for the wealthy by cutting Social Security for our senior citizens."

A spokesman for Mondale noted that Labor Day "is a customary starting point" for fall election campaigns and said the former vice president was "trying to take the occasion to set a sense of tone and a sense of direction for the Democratic critique of Reaganomics in 1982."

In deciding to make the political addresses, Mondale was also borrowing a page from Reagan's own pre-1980 campaign strategy, when he launched a series of regular radio commentaries intended to establish him as the leading Republican spokesman on political affairs.

In his speech yesterday, Reagan also paid tribute to the workers of Poland and celebrated "the strength of our free labor movement."

"In Poland a few days ago, the people peacefully gathered to mark the second anniversary of Solidarity -- a labor movement which revived our hope that non-violent change and basic human rights could come to a closed communist community," Reagan said. "Their parade was met with guns, concussion grenades, tear gas, and water cannons. As we attend our parades and picnics, let us remember how fortunate we are to be a free people."