Are we beginning to see the rationalization of American political parties, long foreseen and sometimes advocated by political scientists, with conservatives clustering in an enlarged Republican Party and the last remaining Republican liberals leaving for the Democratic ranks? Well, maybe. Certainly we're seeing something unusual. Former Massachusetts governor Edward J. King got up early Monday morning to go to town hall in Winthrop and switch parties. The prospect is that Mr. King will run for governor as a Republican next year against incumbent Democrat Michael Dukakis.

That won't be a novel experience. Mr. King beat Mr. Dukakis in the 1978 primary and then, after four years in office, lost to him in 1982. They stand for diametrically opposed policies in theory, though they have converged in practice; Gov. Dukakis will be campaigning on the buoyancy of the Massachusetts economy, which many of his backers will concede is due in large part to the tax cuts championed by Gov. King. On cultural issues, however, they remain far apart. The glib Mr. Dukakis and the painfully inarticulate Mr. King personify the longtime split between gown and town in New England politics.

At the White House Mr. King's conversion was hailed as a sign of the strength of the Republican Party. There's a real possibility that in four of the 10 largest states holding gubernatorial elections next year -- Texas, Florida, Michigan and now Massachusetts -- the Republicans will be running a former Democrat with a good chance to win. In Massachusetts, Mr. King may even have former Democrats running as Republicans for lieutenant governor and attorney general. Former Democrat Ronald Reagan must be cheered at the prospect.

The Democrats' initial response has been to grumble that the party switchers are, as Massachusetts Democratic chairman Chester Atkins said of Mr. King, "self-serving and opportunistic." Of course they are, but then how many politicians don't serve themselves and take advantage of opportunities? Mr. King and the rest fit plausibly enough under the Republican umbrella. The Democrats would do better to boast that the Republicans produce so few talented politicians that they must raid the Democratic farm clubs to fill their big league slots. But this is mostly inside baseball. The party-switching, at the least, will enliven some important political races in 1986, which in turn may tell us whether these switches are symbolic of more general shifts among the voters themselves.