AS STARTLING as the news that Texas Democrat Kent Hance had become a Republican was the announcement last Wednesday in Detroit that Wayne County Executive William Lucas is becoming a Republican. This again is not just a matter of local importance. Mr. Lucas is the chief executive of the nation's fourth-largest county, he has been elected by overwhelming margins in one of the nation's most heavily Democratic constituencies, and he is black. The rumors are floating -- and he is making no effort to discourage them, though he won't confirm them either -- that he will run for governor next year.

Certainly the leaders of Michigan's Republican Party, who have swept the decks of other candidates, hope he will. The incumbent Democrat, James Blanchard, whose popularity plummeted after a tax increase in 1983, has recovered in tandem with the state's economy in 1984, and in the ordinary course of things in this ordinarily Democratic state would be a heavy favorite for reelection. But Mr. Lucas is no ordinary opponent. He has the capacity on the one hand to undercut Mr. Blanchard's base by winning a substantial share of the votes of Michigan's blacks, and at the same time, his record as a budget-cutter and a fighter against crime could make him highly attractive to whites, most of whom usually go Republican anyway.

Like Los Angeles Mayor Thomas Bradley, who came within 1 percent of being the nation's first elected black governor in 1982, Mr. Lucas has a Horatio Alger biography and a professional background in law enforcement. Both men made it into college through athletic scholarships and then joined big city police departments at a time when they were anything but hospitable to blacks. Mr. Lucas then worked for the Justice Department and the FBI, and was assigned to Detroit in 1966; he became Wayne County sheriff in 1970, and was elected county executive in 1982. One of state government's top priorities, he says, is to build prisons to hold 4,000 more inmates; it needs also, he claims, to lower taxes to urge business investment and to reduce the unions' stranglehold on government.

Not quite the platform of, say, Jesse Jackson. Mr. Lucas, who rose in white-dominated law enforcement agencies and has always run for office in majority- white constituencies, evidently feels less uncomfortable with Ronald Reagan's Republicans than with what he regards as the AFL-CIO's Democrats. Some may see his party switch, like Kent Hance's in Texas, as a matter of political opportunism; and if he had seen a clear path to the governorship or other high office under the Democratic label, perhaps he would have taken it. But it does say something about the vitality of the Republican Party that prominent Democratic politicians are now switching parties. And it would say even more, and alter politics in ways not now entirely predictable, if the first black governor in American history turned out to be a Republican -- even a newly converted one.