IT'S as simple as a Frank Capra script. Dukakis has followed the movie star theory of politics, so effective for President Reagan. Voters love a star and the Democrats are giving them one of America's most beloved actors.

What makes the choice so masterful is that Dukakis has crossed party lines to snag one away from the Republicans. Say goodbye to that Massachusetts liberal tag. With Stewart, Dukakis can now lure voters back to the Democrats.

That ain't hogwash.

What can Bush do now? Pick Bob Dole? Name any movie Bob Dole has starred in. Even if Bush does choose an actor to balance out the Stewart factor, who will he turn to? Sly Stallone? Robert Mitchum? Stewart is Hollywood. He is the last of the great film giants, a guaranteed box-office smash.

And what a record.

Ever since Stewart starred in the 1939 classic Mr. Smith Goes to Washington, he has the kind of style Dukakis needs in a running mate. Let Mr. Smith speak:

"I wouldn't give you two cents for all your fancy rules if, behind them, they didn't have a little bit of plain, ordinary kindness--and a little looking out for the other fella, too."

You can see the election motto now: Dukakis-Stewart '88, "Doing a little looking out for the other fella."

Stewart reached the peak of his film career in 1946 with It's a Wonderful Life. What politcian could say that he starred in one of the most classic films of all time? Only Jimmy.

"Every time you hear a bell ring," Stewart could say this week at Atlanta, "it means that some American has just voted Democratic."

DUKAKIS doesn't even have to worry about losing the West now that he has a running mate who was part of the cast for the 1962 film, How the West was Won. All Dukakis has to do now is turn to Stewart instead of paying attention to the polls.

"Golly, Michael, I can tell you every darned thing you want to know. That Bush is pure balderdash. We can lick him up like a tadpole out of the water. Hey, and let's not forget about doing it for the other fella."