Arguing that revising federal exhaust emission laws could boost new car sales by 300,000 a year, executives of U.S. automakers yesterday urged Congress to act quickly on proposed changes in the Clean Air Act.

In meetings with House Republican and Democratic leaders, Detroit's top brass said easing emission controls is the most important thing Congress can do to aid the beleagured auto industry.

Quick action on emission limits is needed because the industry will soon have to make decisions about what kind of pollution control gear to put on its 1983 models, said General Motors Chairman Roger Smith, who this year is serving as chairman of the Motor Vehicle Manufacturers Association.

Smith lead an auto industry team that included Philip Caldwell, chairman of Ford Motor Co., Gerald Greenwald, vice chairman of Chrysler Corp., Gerald Meyers, chairman of American Motors Corp., and James W. McLernon, president of Volkswagon of America.

While the top executives were lobbying in Washington, sales managers in Detroit released figures showing car sales for the first 10 days of December were the worst since 1959.

Ford's sales are down 37.7 percent from last year, AMC is off 27 1/2 percent, General Motors is down 26.4 percent and, even with rebates for buyers, Chrysler sales slipped 14 percent.

The industry sold 125,000 cars during the period, down from 173,000 in the same period a year ago. At this rate, the industry will sell only 5 million 1982 models.

The Detroit delegation made its plea for action on exhaust rules in separate meetings with Democratic and Republican leaders.