Wisconsin Republicans head to the polls today to nominate a Senate candidate, a decision in one of the nation’s most-watched states that could prove crucial in the GOP’s bid to win control of the Senate in November.

The competitive, four-way primary tightened in the campaign’s final weeks, with former Gov. Tommy Thompson losing ground in recent polls to businessman Eric Hovde and former Rep. Mark Neumann, both of whom enjoy tea party support. State Speaker Jeff Fitzgerald is running in fourth and likely to finish last, but he could factor in the contest by pulling votes from the leading the contenders. Sources said the race could be won with 30 percent of the vote or less.

“It’s been very difficult for people to get centered and really understand the Senate race,” Thompson said in a press call hours after Mitt Romney tapped House Budget Chairman Paul Ryan (R-Wis.) as his vice presidential running mate. “It’s never been more apparent or more certain that Wisconsin can become the 51st vote to help the Republicans win the United States Senate.”

“I just want everyone to know that I am the conservative Republican that can win in November,” Thompson continued. The former governor and former Health and Human Services secretary under President George W. Bush is casually refered to as “Tommy” by nearly everyone in Wisconsin.

Democrats and Republicans alike express confidence in winning in November regardless of who wins the GOP nomination, and electability in the general election has been a major argument of Thompson’s throughout the primary campaign, which many considered his to lose based on his high name identification and popularity when in office. Rep. Tammy Baldwin is the presumptive Democratic nominee. Sen. Herb Kohl (D) is retiring.

The addition of Ryan to the national ticket has deflected attention yet again from this key Senate race, just two months after Wisconsin’s political oxygen was consumed by a statewide gubernatorial recall election. Moreover, the intense spotlight on the state hasn’t made today’s outcome any clearer. It’s just raised the stakes.

The Romney/Ryan presidential campaign rally held Sunday in Waukesha, Wis., as a part of the Congressman’s introduction as the running mate only served to make for a stressful, if not awkward, last few days of the Senate primary campaign with no sure Senate candidate to showcase.

Many Wisconsin political observers previously assumed that Thompson had an unshakable base of voters. But Hovde and Neumann haven’t just gained at the expense of each other, they’ve chipped away at Thompson’s support. Neumann has much of the support of Washington’s tea party class, including from organizations such as the Club for Growth, as well as Sen. Jim DeMint (R-S.C.).

Many Republicans, however, believe there is still ill will between him and Gov. Scott Walker from their 2010 gubernatorial primary. Of all the GOP candidates, Neumann might have the most difficult time beating Baldwin in November, at least according to Democrats. One Democratic operative in the Badger State called Neumann “Wisconsin’s Sharron Angle,” in reference to the Republican who lost to Senate Majority Leader Harry Reid (D-Nev.) last cycle in a race many thought would be a GOP pickup.

Hovde has surprised some with the strength of his campaign, particularly because he spent 24 years living in Washington, D.C., before moving back to Wisconsin just recently to run for office. Hovde has spent millions of dollars of his own money on the campaign, much of it on brutal attack ads linking Thompson to President Barack Obama’s health care law.

A poll taken at the beginning of this month by the Marquette University Law School had Thompson at 33 percent, Hovde at 24 percent and Neumann at 21 percent. But a more recent sample from Public Policy Polling, a Democratic-leaning firm, had the race much tighter, with Thompson at 27 percent, Hovde at 25 percent and Neumann at 24 percent. Two GOP sources who had viewed internal polling confirmed that the race is tracking as tightly as the PPP snapshot suggests.

In a last-ditch effort to prove his conservative bonafides, Thompson campaigned Monday with Joe “the Plumber” Wurzelbacher and has been playing up support from Ryan and Walker, even though neither man has endorsed him officially in the race. In the press call Saturday, Thompson called Ryan “a friend” and a partner in policy, “especially on Medicare.”

Though it’s unclear how high turnout will be, especially after more than 2 million voters turned out for June’s recall election, several insiders projected that a higher turnout – 800,000 voters or more – could favor Thompson.

With the Senate in the balance this fall and all eyes on Wisconsin because of Ryan, today’s primary is probably only the beginning of one of the nation’s most contentious races.