The blue Pontiac Lemans and Sen. Edward M. Kennedy (D-Mass.) had been together 10 years. But several months ago the senator decided to call it quits.

Then he waited.

Finally, about two weeks ago, according to an aide, Kennedy's new car came. It was a white Chrysler LeBaron Medallion convertible, with Mark Cross designer leather interior, front wheel drive, a four-cylinder engine, console-shift automatic transmission, power top, front bucket seats and a rear bench.

The LeBaron caught the sun just so. It was elegance sans pretense. In a word, folks, a class act, for about $14,900, an act that is catching on.

In Detroit, Chrysler Corp. officials are beside themselves with joy. In the Washington metropolitan area, where about 1,400 of the LeBaron convertibles and their lesser Chrysler Corp. cousins are on order, Chrysler dealers are filled with glee. After flirting with bankruptcy, Chrysler, the nation's third-largest automaker, has come up with a winner.

The Chrysler convertibles, the first domestically produced top-down cars since the early 1970s, are selling fast--so fast, in fact, that buyers have had to deposit $1,000 and line up on waiting lists for delivery.

Now Ford Motor Co. and General Motors Corp. are hustling to grab a share of the apparently growing market for convertible cars. Ford, the second-largest domestic car manufacturer, plans to introduce a Mustang version this fall. GM, the domestic auto leader, has a Riviera version ready to roll on its Buick line. GM also is considering a Chevrolet entry--probably a Cavalier--in the convertible sweepstakes.

But for the moment, Chrysler is ahead.

It all started in 1980, in the days when Chrysler was cutting staff, closing plants, and taking other measures in a desperate bid for solvency. One move was a cautious commitment to a new convertible.

The plan was to build 3,500 LeBaron Medallions for introduction this spring, but when the company started advertising the convertibles last January, it received orders for 3,800 LeBarons within a few days, said John Givens, Chrysler's vice president for marketing.

The orders kept coming and the company decided to add a medium-priced LeBaron and a Dodge 400 series--both of which list for about $12,600--to its convertible line.

Chrysler now plans to produce and sell 20,000 convertibles by the end of the 1982 model run in July.

The growth in demand changed Chrysler's new convertible production from a job-shop type of operation to the familiar full-scale-assembly-line approach.

"When we started out, we had figured on only a small volume of cars, which would have been impractical for full in-house production," Givens said. "We don't know how long this demand will last, but we're very enthusiastic. It's now become a full production kind of thing," Givens said.