In a state that has hosted two rounds of historic recall elections in the last year, Tuesday’s primary might seem like an afterthought.

But a Senate primary season that has already featured Republican upsets in Nebraska, Indiana, Missouri and Texas might make room for one more in the Badger State. And it just might be the most consequential one of all.

Former Wisconsin governor Tommy Thompson greets supporters after formally launching his bid for U.S. Senate. (Dinesh Ramde/Associated Press)

Since the Republican race kicked into gear last year, Tommy Thompson, a well-known former governor who served four terms in the state’s top job, has been the GOP front-runner.

But in the race’s closing weeks, Thompson’s lead has grown tenuous. And the combination of a spirited effort from national conservative groups who despise him and the viability of two alternative Republican candidates could be enough to prevent the former governor from celebrating Tuesday.

Both the Club For Growth and Sen. Jim DeMint (R-S.C.) are backing former Congressman Mark Neumann, arguably the most conservative candidate in the field. Supporters of Neumann have slammed Thompson’s past moderate positions, including support for an individual health-care mandate. Meanwhile, Eric Hovde, a self-funding political newcomer from the private sector, has been flooding the airwaves in ads for months. His strategy has brought him well within striking distance of the lead.

Conservatives may not like Thompson’s record, but he sure looks like the most electable Republican. An early August Marquette Law School poll showed Rep. Tammy Baldwin (D) leading or tied with all Republican candidates except Thompson, whom she trailed by four points. A recent Quinnipiac University/CBS News/New York Times survey showed Baldwin leading all Republican candidates except Thompson with whom she was tied.

In the Republican primary, Thompson’s lead has shrunk. An 18-point advantage in a June Marquette poll has dwindled to a 33 percent to 24 percent lead over Hovde in the latest survey, with Neumann surging up to 21 percent.

Why would a Thompson defeat be so notable? So far, the upsets of note in Republican Senate primaries have come in red states Mitt Romney is expected carry at the presidential level in November.

Ted Cruz’s win in Texas, surprising as it was, doesn’t make the Lone Star State any less a GOP lock in November. Deb Fischer’s win in Nebraska was a blow to the state GOP establishment as well as the Club For Growth. But she remains a substantial favorite over former Democratic senator Bob Kerrey. Republican Richard Mourdock’s toppling of Sen. Richard Lugar (R-Ind.) prompted a Democratic celebration. But Rep. Joe Donnelly (D-Ind.) is still in the unenviable position of having to run ahead of President Obama, who is expected to lose Indiana.

The most significant upset to date has taken place in Missouri, where Rep. Todd Akin (R), aided by a Democratic effort to boost his candidacy, won the Republican nomination last week. His uncompromising social conservatism and unpredictable streak gives Democrats a bit more hope than they would have had against other Republicans. But polling conducted toward the end of the primary still showed Akin leading Sen. Claire McCaskill (D). And McCaskill won’t be benefitting from the top of the ticket, as Obama isn’t expected to win the Show-Me State.

Wisconsin, however, is more of a swing state. It hasn’t voted for a Republican presidential nominee since Ronald Reagan in 1984. But in 2000 and 2004, George W. Bush came within a point of victory. And in 2010, Republicans took control of the governorship and state legislature. They hung onto the former in a June recall election.

Why is Thompson the strongest Republican for the general election? For starters he’s well-known across the state. (You don’t serve four terms as governor without leaving a lasting impression.) Secondly, the same perceived moderation that is hurting him in the primary could help Thompson with independents in the general election.

To be clear, neither Hovde nor Neumann would be general election disasters. And in Wisconsin’s polarized environment, GOP energy should be high regardless of the nominee.

However, in a general election race expected to be close, Thompson looks like the best GOP bet. That simply can’t be overlooked.

Hovde is a still an unknown quantity. And he’s lived in Washington for much of his adult life. Meanwhile, Neumann’s conservatism makes him less of a threat to win the ideological middle, and he has a yesterday’s news quality about him, as he’s lost statewide bids more than once already.

The polling speaks for itself, and so does the Democratic posture, which appears to confirm a preference to run against Neumann, not Thompson. The Democratic-aligned Majority PAC released an ad in July which hit Hovde and Thompson, but spared Neumann.

Regardless of Tuesday’s outcome, both sides will claim to have the upper hand Wednesday morning. Allies of Baldwin, whose fundraising has been very strong, will say that the bloody Republican primary has weakened the GOP nominee. Republicans will point to Baldwin’s liberal record in the House as a reason they relish challenging her.

Tuesday will test whether Republican voters prize electability for what promises to be a competitive fall campaign. Thompson is sure hoping that they will.