IF POLITICS IS THEATER, then Ted Kennedy is its premier actor. He can make you laugh, he can make you smile, he can conjure up memories, and -- if you're one of his supporters -- he can break your heart. He can do everything but win.

He can, as he did last night, take a convention and lift it right out of its seat. He can focus the debate, finally, on the Republicans by conjuring up FDR, and by taking Democrats back to their roots. It is Ted Kennedy who seats you by an old radio and makes you hear the voice of Franklin Roosevelt: "Can the old guard pass itself off as the New Deal?"

What he did last night was provide a blueprint for the coming attack on Ronald Regan. He took the Republican nominee's own words and speared him with them. He cited Reagan's attack on Social Security -- his suggestion that it be voluntary -- and with it summoned up the old specter of the unfeeling Republican in a high hat.

He talked of a Ronald Reagan who "is no friend of the senior citizen," who once said of the environment that "80 percent of air pollution comes from plants and trees." He recalled Reagan's sentiments about New York City -- how he said he "prayed" that the federal government would not bail it out.

"And that nominee, whose name is Ronald Reagan, has no right to quote Franklin Delano Roosevelt," Kennedy said, referring to Reagan's acceptance speech. Madison Square Garden went wild.

From there, Kennedy outlined his prescription for a Democratic America -- an America of full employment and universal health insurance and low inflation. He talked of Liberty City, the section of Miami struck by recent riots, and of the South Bronx, a neighborhood gone dead, now nothing but rubble and broken people.

Kennedy graphically attacked the GOP tax program, saying it would greatly benefit only the people in this country who make $200,000 a year. The convention seemed stunned at that figure. And when he came to the Equal Rights Amendment, his support of it was so strong, so clear, so unequivicable that even Carter delegates, who had sat stone-faced and unmoved until then, rose and cheered.

This is a lost art, this ability to make a rousing political speech, and it is possible that Kennedy is its last practioner. If you had to reach back for a similar speech, it would have to be Kennedy's to the 1972 Democratic convention. Further back than that, you would have to look to Bobby Kennedy and then Jack. If there have been other great political speeches of recent times, they must have been made by a Kennedy. It must be something that runs in the family.

Speaking last night, Kennedy somehow took the focus off himself and onto his party. He somehow succeeded in making himself and the party one, as if he personified it. He stood there on the platform as Mr. Democrat, the repository of all the old ideals -- New Deal, Fair Deal, New Frontier, The Great Society. Everything after that, he seemed to say, lacked soul.

The speech touched off a demonstration. It made people cheer and it left even the cynical, if not moved, impressed. You don't get to see and hear this type of thing anymore.

There has to be a place in politics for theater. There has to be a place in politics for emotion. There has to be a place for sentiment, for a tear, for remembering FDR and Harry Truman and Jack and Bobby. There has to be a place for ideals and rhetoric, for fine phrases and ringing words and little poetry at the end. There has to be a place for a Kennedy.