INDIANAPOLIS — In the high-stakes race for the U.S. Senate, Indiana’s major-party candidates and their supporters have waged aggressive and costly campaigns to woo voters like Aaron Williford.

The 37-year-old “likely voter” hails from a traditionally Democratic area, Kokomo, which benefitted from the auto-industry bailout opposed by Republican candidate Richard Mourdock. But he’s also a fiscal conservative who worries that government-spending plans supported by Democratic candidate Joe Donnelly aren’t sustainable.

In mid-October, Williford met both candidates when he was picked by the Indiana Debate Commission to pose a question of his choice during the first televised Senate debate.

His question, about the proper role of the federal government, prompted one of the sharper verbal exchanges between Donnelly and Mourdock – with each accusing the other of distortions and lies.

Did Williford leave feeling better informed? “Not so much,” he said.

Instead, he left the debate seriously thinking about voting for the third candidate on the ballot: Libertarian Andy Horning.

The race for the U.S. Senate seat has been filled with the unexpected, beginning with the knock-out punch delivered by the tea party-backed Mourdock in the May primary to the six-term GOP incumbent, U.S. Sen. Richard Lugar.

Another surprise came in late September as Republicans had virtually locked in the race for governor and clinched the state for GOP presidential candidate Mitt Romney. That’s when the independent Howey-DePauw poll found Donnelly and Mourdock in a statistical dead heat (with Donnelly up 2 points over Mourdock) but with 7 percent of the voters saying they’d pick Horning.

Other pollsters since then have put Mourdock ahead, but not by much, prompting speculation that Horning could be the “spoiler” candidate – a label he finds dismissive and offensive.

“It’s not their race,” Horning said of his major-party opponents. “It’s up to the voters to decide.”

The tight race has captured the attention of national political watchers because the outcome is significant beyond Indiana. It’s one of a handful of races across the U.S. that could tip the balance of power in the Senate from Democratic to Republican.

Republicans would need to pick up four new seats and hang onto to all their current seats to win control of the Senate next year – and Indiana is one of the seats the party can’t afford to lose.

That’s why so much money is flooding into the state. Spending on the race had topped $20 million by the third week of October, with huge chunks of that coming from national Republican and Democratic organizations and so-called “super PACs” – political action committees that are allowed to raise and spend unlimited amount of money on political advertising.

Any Hoosier with a TV likely already knows that. According to data compiled by Kantar Media’s CMAG – a non-partisan tracker of media ads – more than 22,000 political ads related to the Senate race aired between mid-July to mid-October. And more are on their way.

“This is first time we’ve seen really large amounts of money from outside forces getting spent here,” said Andy Downs, director of the Mike Downs Center for Politics at Indiana University-Purdue University Fort Wayne.

Downs said all that outside money had escalated an already volatile race between  Donnelly, a three-term congressman, and Mourdock, the state treasurer. And it’s resulting in some campaign ad attacks that are more over-the-top than what Hoosiers may be used to seeing, he said.

“The job of these super PACs is to win,” Downs said.

On the campaign trail, Donnelly and Mourdock have each tried to define themselves as nice guys who are independent thinkers.

“I’m just Joe,” is how Donnelly, 57, described himself during the first debate, urging voters to cast aside political party labels and embrace him as the bipartisan-loving candidate. The South Bend congressman is more conservative than some of his Democratic colleagues on Capitol Hill: He’s opposed to abortion, supports gun rights and thinks a balanced budget amendment is a good idea.

Mourdock, 61, beat Lugar by 20 points in the May primary by convincing Republican voters that the 80-year-old senator had compromised too often and too much with the other side. After his primary win, Mourdock, of Darmstadt, said “bipartisanship ought to consist of Democrats coming to the Republican point of view.”

Horning, the Libertarian, calls both men “cogs in the machine” of a corrupt federal government.

Both Mourdock and Donnelly have tried to woo “Lugar Republicans” – GOP voters who were unhappy with Mourdock for Lugar’s loss.  Both candidates have taken to calling Lugar an American hero.

A pro-Mourdock campaign flyer recently put out by a super PAC made it look like Lugar was supporting Mourdock. In a strongly worded response, a Lugar spokesman made it clear that Lugar had no plans to campaign for Mourdock. But he wasn’t backing Donnelly either.

Brian Howey, publisher of Howey Politics Indiana, said the race might go down to the wire, as voters split their tickets. 

Indiana voters have a history of doing so, Howey said. In 2008, Indiana went blue in the presidential race to help put Barack Obama in the White House, while re-electing Republican Mitch Daniels to a second term. In 2004, Indiana supported Republican George W. Bush over John Kerry by more than 20 points while voting to send Democrat Evan Bayh back to the U.S. Senate with a big vote margin. When Indiana voters went for Bush in a big way in 2000, they also elected Democrat Frank O’Bannon as governor.

• Maureen Hayden covers the Statehouse for the CNHI newspapers in Indiana. She can be reached at maureen.hayden@indianamediagroup.com.