An unlikely Democratic hero is Stephen V. Monsma -- a soft-spoken but firm abortion foe and tax cut advocate who wears his longtime affiliation with the party of Walter F. Mondale conveniently under his sleeve.

But many disheartened Democrats are looking to the lanky college professor to score an upset in Tuesday's special state Senate election here that would give them a prestigious victory in heavily Republican west Michigan -- with a generous fringe benefit.

If Monsma defeats fellow Dutchman, fellow professor and fellow Calvinist Church elder Vernon J. Ehlers, the Republican nominee, Democrats would regain a numerical majority in the state Senate.

They lost that status last year after two Detroit-area Democratic senators who supported a controversial 38 percent state income tax increase were recalled by their constituents and replaced with Republicans, who now hold a 19-to-18 edge. Lt. Gov. Martha Griffiths, the tie-breaking voter, is a Democrat.

There are national implications as well to this $600,000 race, Michigan's most expensive state Senate contest. Democrats are governors in two-thirds of the states and control two-thirds of the state legislative chambers as well.

Republicans, however, gained 320 legislative seats in last year's contests -- including five in the Michigan House that deprived the Democrats of a dependable working majority there, according to House Speaker Gary Owen (D) of Ypsilanti.

Nationally, Republicans are scrambling to transplant their successes in recent presidential elections to the increasingly important soil of state and local politics.

"The next big battleground in American politics is going to be the statehouses and the state legislatures," 1988 presidential hopeful and former Senate majority leader Howard H. Baker Jr. (R-Tenn.) told a crowd of 1,400 at a GOP fund-raising dinner here last week.

Among the most important spoils in that battle will be the upper hand in redrawing congressional district lines after the 1990 census.

Moreover, Monsma seems to offer a competitive alternative for the Democratic Party at a time when it is trying to shed its image as a ship of special interests aground on the left bank of the American mainstream. Polls from both sides suggest a close vote Tuesday, with Ehlers a slight favorite.

To win, Monsma strategists say they need an enthusiastic turnout from dependable Democratic voters, not all of whom have warmed to a Monsma campaign that takes traditionally non-Democratic stances on some favored issues.

The prize being sought by physicist Ehlers, 51, and political scientist Monsma, 48, is to represent the Senate district centered in Grand Rapids (population 181,104), Michigan's second-largest city and the home town of former president Gerald R. Ford.

The Senate seat has been one resting place in a local game of political musical chairs. Republican Paul Henry held it until he was elected to Ford's one-time seat in Congress last year. Henry won the Senate seat in 1982, after Monsma gave it up to run unsuccessfully for the Ford congressional seat, then held by Republican Harold S. Sawyer.

Ehlers, former chairman of the Kent County Board of Commissioners, has been a state representative since 1983, representing the southeastern half of Grand Rapids -- somewhat to his political detriment in this campaign.

Monsma has been telling fellow Dutch voters, who make up about one-fourth of the local electorate, that they can double their influence in the legislature by electing him to the Senate and leaving Ehlers in the House. Some Ehlers supporters call that the "two Dutchmen" argument, and concede that it is having some effect.

Republicans hold a 5-to-4 edge among the Senate district's 140,000 registered voters. But both sides expect a low turnout, with an estimated 40,000 voters likely to take part.

Ehlers and some of his key supporters acknowledge that Monsma's long-held conservative style and stance on many issues are frustrating their efforts to paint him as a "liberal Democrat" and virtual lockstep supporter of organized labor.

Both candidates oppose abortion, but the political action committee of Right to Life of Michigan, the state's major anti-abortion group, endorsed Monsma over Ehlers by a 12-to-10 vote, diluting a usually certain source of votes for Republican candidates.

Despite that endorsement, Monsma will probably draw votes from both sides of the issue, Ehlers lamented, because "most of the pro-choice people tend to be liberal, and Steve is a liberal."

One GOP tactic that is working is the "balance" issue, in which Republicans, including the state party in its own $200,000 campaign, argue that it is best not to allow the Democrats to control the statehouse, through Gov. James J. Blanchard, and both legislative houses.

The Grand Rapids Press, the city's only daily newspaper, made that argument in its endorsement. "A vote for Mr. Monsma is a vote for a fine legislator," the paper said. "A vote for Mr. Ehlers is a vote for a fine legislator and for a balanced and vigorous legislature. On that basis, we endorse Vern Ehlers."

The fight for control of the legislature is tinged with racial overtones, because some Republican supporters have argued that if their party loses, power will pass to senior Senate Democrats largely from the eastern part of the state. Detroit is the political center of that area of Michigan, and most of its population, as well as its feisty mayor, Coleman A. Young, are black.

Rick Wiener, Michigan Democratic chairman, called it an "all-win" situation for his party, with the real test to come next year, when Blanchard seeks reelection. But Rep. Henry (R-Mich.) disagreed. "The whole issue at this point would really be momentum," Henry said. "The momentum is in our direction, and you just don't want to break it."