MADISON, Wis. -- For the last few years, the most endangered species in Wisconsin appeared to be the liberal Democrat. In one election after another, unhappy voters tossed them from public office, defeating long-serving incumbents, rebuffing promising newcomers and capping it off by creating a new national conservative hero, Republican Gov. Scott Walker.

For a state that was the home of Earth Day founder Sen. Gaylord Nelson and long epitomized modern liberalism, it was quite a turnabout.

But now it appears the surprises may not be over.

Despite the conservative swing, one of the most liberal politicians in the country is now suddenly locked in a tight race for a Wisconsin Senate seat with a Republican who supposedly couldn't lose, a man so well-known that most people call him simply by his first name: Tommy.

What's up with Wisconsin and its Senate race is a question now being weighed by both political parties, as well as major interest groups, which are spending millions of dollars in an effort to swing the voter mood toward either Democrat Tammy Baldwin or Republican Tommy Thompson in the final frantic weeks before Election Day.

A Marquette University Law School poll released Wednesday showed the race tied, with Thompson up 46 percent to 45 percent. The gap was within the poll's margin of error.

Adding importance is that the race will help determine whether Democrats maintain majority control in the Senate. The seat is open due to the retirement of Democratic Sen. Herb Kohl.

In recent years Wisconsin has been at the center of a political storm that moved most of the Midwest politically to the right, driven by public angst about the recession and a sense that government was spending too much. In the 2010 midterm, Wisconsin voters replaced veteran Democratic U.S. Sen. Russ Feingold with tea-party Republican Ron Johnson, gave the Legislature to the GOP and launched Walker on a spree of cutting budgets and weakening union power.

After Kohl announced his retirement, the GOP seemed ready to make his seat its next trophy. But the race has gone off script, even though the Democratic candidate, Baldwin, a gay woman from liberal Madison who had served in Congress for 14 years, appeared to represent everything that many voters were rejecting.

Apparently, Wisconsin is still in flux.

"Whoever wins in an election in a given moment, there's this tendency to assume that Wisconsin is actually moving in that direction and that's always been wrong," said Mike McCabe, head of a government watchdog group the Wisconsin Democracy Campaign. Actually, "sometimes people will give one side a try, but if they're less than impressed with the results, they'll give the other side a try."

Factors behind the new mood, according to those in the campaign, could include everything from Baldwin's youthful contrast to Thompson and her early financial advantage in the race, to the battering Thompson took in a tough primary with tea party opponents, to the possibility that voters are just feeling better and are more receptive to Democratic appeals to the middle class.

The 70-year-old Thompson, who was elected governor four times in the 1980s and 1990s, admitted being exhausted after the four-way primary in August, in which his younger, more conservative opponents argued that his time had passed.

At one point, determined to show his energy, he dropped down and did 50 push-ups in front of a newspaper's editorial board.

Still, some conservative voters are wary that he's really in synch with a Republican Party led by new leaders like Walker and vice presidential candidate Paul Ryan, who advocates replacing Medicare with a voucher system. As governor and later as Health and Human Services secretary, Thompson defended health programs and sought consensus with Democrats. In the primary, both the national Club for Growth and the Tea Party Express ran ads against him.

Nancy Milholland, an organizer of the Racine tea party group, said most conservatives are still willing to vote for him.

"It's very few on the outer edges that whine and complain," said Milholland.

Baldwin, 50, is also pushing the argument that Thompson's time has passed.

In campaign appearances, Baldwin exudes a soft-spoken friendliness and sense of empathy that helps soften any image of a strident liberal.

Like Thompson, she has spent most of her adult life in public office, in her case since being elected to a county board when she was 24. If elected, Baldwin would become the first openly gay U.S. senator, though her orientation hasn't been an issue in the campaign.

Jean Lynner, a Republican voter from Waukesha, said she's not keen on anyone as liberal as Baldwin, who supported President Barack Obama's health reform plan and federal stimulus program. But she also has problems with Thompson, who she described as "an old politician" from an era of "good ol' boys clubs and backroom deals."

Thompson hopes to gain momentum on the campaign trail. He works crowds enthusiastically and thanks people by name in folksy style that quickly gets them smiling.

At a recent stop at Busch Precision in Milwaukee, Thompson spun stories about conversations he had with the company's founder more than 20 years ago and dropped names of people in the crowd.

"I'm not going to retire," he said, motioning toward the crowd. "Our best days are in front of us, Audrey."

Baldwin outspent Thompson on television advertising 3 to 1 in the race's initial weeks, helping to fuel her surge, but Thompson has raised $2.2 million since the primary, and benefited from millions spent by outside groups including Karl Rove's Crossroads GPS. That group put another $1.2 million into television ads on Wednesday, bringing its total to $5 million.

Both sides are now saturating Wisconsin media. About $19 million has been invested by outside groups alone in the race, second only to the sum spent on Virginia's Senate race, according to the Center for Responsive Politics.

Pro-Thompson forces hope there's still more liberal phobia in the electorate to mine. Their ads hammer Baldwin as to the left of both Obama and former House Speaker Nancy Pelosi -- "too extreme for Wisconsin."

The pro-Baldwin side counters by portraying Thompson as one of the detached elite -- "not for Wisconsin any more" since he made millions in the private sector in Washington after leaving government in 2005.

Even though polls indicate Wisconsin voters are nearly evenly split between Obama and Romney in the presidential campaign, they could wind up choosing candidates in other races with seemingly contradictory views, said McCabe. "This is a very schizophrenic state politically and always has been."