Primaries forcing front-runners to sharpen campaigns; On to Michigan, where Jackson's strength may undermine labor's hopes for Mondale

It's just shy of 6 a.m. and Steven Buchanan stops a moment to look his questioner in the eye. ''Mondale,'' he says decisively. ''Hart? I think someone just fell in love with him.

''I'd like to see Jesse Jackson (for vice-president),'' says Mr. Buchanan, a black union auto worker heading into the Chrysler plant here on East Jefferson Avenue. But ''Mondale has the experience.''

Buchanan is an example that, sometimes, the organization works.

After Tuesday's primaries in the South and East - where Democratic presidential hopefuls Walter Mondale, Gary Hart, and the Rev. Jesse Jackson each could claim some kind of victory - the political spotlight has turned to Michigan.

Its Democratic caucuses Saturday will be the first indication of voter sentiment in the industrial Midwest. At stake in Michigan is an important chunk of delegates - 155, to be exact. And the state will provide a proving ground, not only for the candidates, but for the cohesiveness of the two most important voting blocs here:

* Labor. Unions, especially the United Auto Workers, have pushed very hard on their home turf. This heavily unionized state is where big labor will prove whether it can deliver votes to Mr. Mondale, observers say. ''I can't remember us working as hard as (on) this one,'' says Stephen P. Yokich, director of the Michigan community action program for the UAW.

* Blacks. Traditionally, they are the second-most-important group in the state and the most homogeneous voting bloc, according to observers. But this round is a case of divided loyalties between Mr. Jackson, the emotional favorite , and Mondale, the favorite of establishment figures, most notably Detroit's black mayor, Coleman Young.

Since labor in other states has so far failed to turn out big margins for Mondale, the key may rest with the number of black votes for Jackson, says Michael Traugott, senior study director for the Center for Political Studies at the University of Michigan. His poll of Democrats and Independents in early March showed that among union workers, Mr. Hart and Mondale were running neck and neck. Overall, his soundings showed that Hart had surged from 4 percent in February to 30 percent, which was just eight percentage points behind Mondale.

Mr. Yokich of the UAW, however, says Mondale did well in Tuesday's primaries. And he is confident of a Mondale victory in Michigan. If Jackson hadn't been in the Georgia race, for example, most blacks would have been in Mondale's camp, he says.

But Jackson is very much in the race and is campaigning this week in Michigan. If, as Mr. Traugott's figures show, white voters split evenly between the two, the black turnout will be the key.

''Blacks are having to choose between two favorites,'' says Robert G. Newby, assistant professor of sociology at Wayne State University.

So far, that key seems to be within reach of Mondale's grasp.

''There are so many roadblocks here,'' laments Sam Riddle, Jackson's campaign strategist for Michigan. ''It will be the most difficult state for Reverend Jackson.''

Big labor, party bosses, and black elected officials are rooting for Mondale, he says. Even the state's caucus system is rigged in Mondale's favor, he adds.

Mr. Riddle, Jackson himself, and others have filed suit in United States District Court to delay the caucuses until their grievances are heard. Among other things, they complain there are not enough polling places, black voters are not proportionately represented, and because balloting is not secret, city and union workers will be reluctant to vote against their leaders, who are pushing for Mondale.

The drama will be played out in Michigan's Democratic power corridor - a swath of union and black voters stretching from Bay City in the north to the southern suburbs of Detroit. A handful of Chrysler workers interviewed at the Jefferson Avenue plant, for example, invariably said Mondale was their man. But almost as consistently, they said Jackson was their choice for vice-president.

A survey of Detroit precinct leaders in January seems to confirm that sentiment, according to Sam Eldersveld, the political science professor at the University of Michigan who conducted the survey.

Saturday's caucuses in Kentucky, meanwhile, have attracted far less attention. The real fight for Kentucky's 53 delegates won't begin until the end of the month. Even then, the governor, this year's convention chairwoman, will try to keep state delegates uncommitted.