DETROIT -- Three critical Michigan voting blocs -- labor, blacks and Reagan Democrats -- are up for grabs Tuesday in a key Rust Belt presidential primary that could all but settle the nomination races in both parties.
All three groups are without their favorite candidate on the ballot and are the focus of lively competition for their support, particularly among the Democrats -- Gov. Bill Clinton of Arkansas, the front-runner in the polls, former Sen. Paul E. Tsongas of Massachusetts and former Gov. Edmund G. "Jerry" Brown of California. But the two Republicans, President Bush and Patrick J. Buchanan, are also contending for the Reagan Democrats in a state where crossover primary voting is easy.
Labor voters, hurting from the decline of the auto industry here, are without Sen. Tom Harkin of Iowa, the pro-labor liberal who dropped out after disappointing showings.
Black voters, who turned out in record numbers in 1988 in behalf of the Rev. Jesse L. Jackson, won't find him on the ballot Tuesday.
Many of the so-called Reagan Democrats, who left their party in droves for Ronald Reagan in 1980 and 1984 and stuck with his hand-picked heir, George Bush, four years ago, have lost their enthusiasm for both Mr. Bush and the Republican Party. So they are fair political game for all the candidates running in both parties.
All this offers hope of one sort or another to each of the three Democrats. Mr. Clinton, who has yet to win outside his native South, has made a strong appeal to blacks as a Southern champion of civil rights who has produced results in achieving racial harmony in Arkansas.
At the same time, he has wooed the Reagan Democrats, offering them a changed Democratic Party that will address needs of the middle class. A tax cut for this group is a centerpiece of the Clinton agenda.
On successive days last week, Mr. Clinton first went to Macomb County, a lily-white, blue-collar stronghold bordering on heavily black Detroit, and then to a large black congregation in the city, calling in both places for an end to racial discord and suspicion.
Mr. Clinton's chances with labor are less certain, because he does not pass some critical litmus tests.
Arkansas has remained a right-to-work state throughout Mr. Clinton's 11 years as governor. He has said he won't fight for repeal of the federal right-to-work law but will sign it if passed by Congress. He also supports "fast-track" trade negotiations with Mexico favored by President Bush that labor says will cost American jobs.
Mr. Tsongas also supports fast-track negotiations. And he opposes legislation sought by organized labor to require the rehiring of strikers rather than keeping on strikebreakers. He argues that the only way to labor peace in which jobs can be created is forced arbitration -- a red flag to unionists.
Mr. Brown may have the strongest labor record of the three, and some union leaders have talked about backing him as a means of keeping the Democratic race unresolved.
They would like to see an open convention in which labor delegates could be important brokers. But to get to the convention, union members need to back a candidate or elect uncommitted delegate slots, not a strong likelihood.
As for Mr. Tsongas, who has been clearly out-organized by Mr. Clinton, his hopes rest not only on winning suburbia and luring Reagan Democrats back into the fold but also on attracting moderate Republicans, who once dominated this state and may be disillusioned with Mr. Bush and turned off by the bombast of Mr. Buchanan.
Mr. Buchanan also needs strong support among the Reagan Democrats, among conservative Republicans who backed Jack F. Kemp and television evangelist Marion G. "Pat" Robertson here in 1968, and from voters hard hit by the state's 9 percent unemployment who blame Mr. Bush.
The polls here, however, show Mr. Buchanan trailing the president by nearly 3-to-1.