When the primary campaign began here last winter for the congressional seat vacated by David A. Stockman, Mark Siljander hardly stood a chance. All the experts agree on that.

Siljander, a 29-year-old "moral conservative," was competing for the Republican nomination against John Globensky, a senior GOP leader whom Stockman, now director of the president's Office of Management and Budget, personally had chosen to be his successor. The party establishment is southwest Michigan backed Globensky with endorsements and contributions; Stockman and Rep. Jack Kemp (R-N.Y.) came to campaign for him.

"It was tough, because Stockman is a near diety here," Siljander says. In the March 24 GOP primary, however, Mark Siljander emerged on top -- because, he says, he enlisted an ally even more powerful than President Reagan's budget director.

"I believe God wanted me in," says Siljander, who is heavily favored to win the House seat in this strong Republican district in the general election Tuesday. "It was nearly a supernatural win. With so many liabilities against us, that's the only explanation."

Siljander says he won divine preference because "I recognize the moral side of the issues."

On economics, Siljander says, he joins most people here in supporting Stockman's plan to cut spending and stimulate business through tax reductions. s

"The difference is that I feel a good part of our economic woes are because of social woes," the candidate says. "We can talk about secular humanism, abortion, promiscuity, child abuse, homosexuality -- general moral-type issues."

Politicians here say Siljander got an outpouring of fundamentalist Christian support that made the big difference in the primary election. Church groups are working to turn out the vote for him Tuesday, a phenomenon that brings dismay and hope to his opponents.

"I've been working against odds all my life, but I never had to work against the Lord," says Democratic nominee Johnie Rodebush, a lanky 58-year-old welder whose wry wit and soft country twang are reminiscent of Will Rogers.

Still Siljander's "moral-type issues" have given his opponents their only opening in a district blanketed by conservative Republican sentiment.

"I don't see that Mark's issues are the important ones," Rodebush said at a joint appearance here last week. "When you're 65 years old and cold and hungry, you're really not too worried about being pregnant or sex education."

"It's risky to mix religion and politics," chimed in Libertarian Bette Erwin. "The government ought to leave religion to individuals."

Bruce Lee, Rodebush's campaign manager, says "the only way we can win is if people get scared about Mark saying God picked him, or Republicans get mad when somebody says it's a sin not to vote for him."

Newspapers here say there is some Republican resentment against Siljander, and a Republican named John Lauve has mounted a write-in campaign to challenge the party's nominee. Siljander notes that some hard feelings follow any tough primary campaign.

But, recognizing the possibility of a backlash, Siljander has started playing down his church support. He stopped wearing his "Jesus First" button, and he tells audiences he should be elected "for one reason: to help Dave [Stockman] get his program through."

Except for Erwin, the Libertarian, nobody here seems to be challenging the Reagan/Stockman program. After all, the farmers and factory hands of this rural district gave both Reagan and Stockman victory margins greater than 70 percent last November. (Stockman was named budget director two months after winning a third House term).

The only debate is how much fealty to Reagan is required from the district's representative. Siljander promises to back the president all the way. Rodebush says he supports "the bottom line of the cuts," but would critically examine it constituent parts.

Even within this narrow range, the debate here reflects the historic conflict of principle between the two parties. At the Sturgis debate, both the Republican and Democrat offered comments on assistance to the poor. See if you can tell which party is speaking:

"When I see seniors going into that [county] courthouse in Kassapolis with tears in their eyes because they need money just to pay the heat bill, I say we've got to help people."

"Government has given people too much. That incurs laziness . . . there is a disincentive to hard work."

The first speaker was Rodebush, the Democrat. The second was Siljander, the Republican who will most likely emerge Tuesday as the winner of the first special congressional election of 1981.