Democratic Sen. Claire McCaskill is confronting strong headwinds in her bid for a second term, and it's made Missouri one of the most watched states on the political map.
But the battle inside the Republican Party over who will face her in the fall has flown largely under the radar.
Indeed, as GOP primaries go, it had been rather sedate. Except for the occasional attack over who's the "career politician," or one of the candidates hoping to "punch Claire McCaskill out," there have been few fireworks.
At least until last week, when a campaign ad from John Brunner -- one of the three leading contenders for the Republican nomination and a former St. Louis businessman -- aired a tough televised ad attacking his rivals. It accused Sarah Steelman, a former state treasurer and state senator, and U.S. Rep. Todd Akin of casting votes that sent Missouri and the country into debt.
It's a potentially powerful charge in a campaign where the economy is the No. 1 issue, and cutting spending is a driving force in conservative Republican politics and the tea party movement. All three candidates are appealing for tea party support.
But more than anything, the Brunner ad introduced a long-awaited jolt to an otherwise somnambulant GOP campaign.
"It's finally injected some energy," said Peverill Squire, a professor of political science at the University of Missouri. "You've got three candidates fighting over the same group of voters, and no one has a strong claim on any of them. They're going to have to work hard to distinguish themselves. We may start seeing the campaign get considerably nastier."
Election watchers have predicted that the intensity would increase once the race got inside the two-month window of the Aug. 7 Missouri primary. Until now, the campaign has been pretty much all about McCaskill, and for good reason.
She's an embattled incumbent in a year when voters are disgusted with the capital's inability to solve problems and the Washington "outsider" label resonates. She's also a close and prominent ally of President Barack Obama, and outside groups are spending millions of dollars to defeat both of them.
Also hanging in the balance is whether Democrats can hold on to their majority in the Senate.
The National Journal calls the race a "coin toss." But against what even some Republicans have acknowledged is a weak field, McCaskill can't shake the "endangered" label. Though a recent Democratic poll gave her single-digit leads over all three GOP hopefuls, her support was below 50 percent.
"They are all competitive," said Jennifer Duffy, a Senate analyst for the nonpartisan Cook Political Report. "Her task is not unlike Obama's, which is to make whoever the nominee is a lot less acceptable."
Whether that turns out to be Steelman, Akin or Brunner, the GOP standard-bearer is likely to face a fierce advertising assault from the Democrats almost immediately.
In the meantime, they're nearly in a three-way tie, with Brunner having gained the most ground since January, according to a survey by Public Policy Polling, a Democratic-leaning survey company.
Eight Senate candidates in all will appear on the Republican primary ballot. The others are: Jerry Beck, Mark Memoly, Mark Patrick Lodes, Robert Poole and Hector Maldonado.
A wealthy first-time office seeker, Brunner has been the only one of the three leading contenders to broadcast ads so far. The others probably are saving their money for the final push. He's attacked both McCaskill and Obama and run positive spots to introduce himself to voters.
The new ad was an abrupt departure from the script.
"He thinks it's to his advantage to be negative, but my sense is that the public got a pretty good snoot-full of that in the presidential primary with everybody ripping each other to pieces," Akin said.
Steelman spokesman Patrick Tuohey pointed out that as a businessman Brunner took on debt at Vi-Jon Inc., the St. Louis personal care products company that he used to own and run.
"It may telegraph the nature of the ads to come from the Brunner campaign," Tuohey said.
The ad claims that while Brunner was "manufacturing products," Steelman and Akin were "manufacturing debt." It cited Steelman's votes for state revenue bonds and certain appropriations bills, and Akin's support for congressional earmarks and votes to increase the debt ceiling.
However, a vote to raise the debt ceiling does not mean spending more money; it's a vote to actually pay the bills for what Congress has already purchased.
Steelman and her supporters said the ad was "false" and a "smear." Brunner officials stood by it, offering voting records and media accounts as backup. They also denied that the ad was negative.
"No grainy, unflattering photos, no ominous music," said Brunner campaign manager John Seaton, who also downplayed its significance. "There've been plenty of barbs already."
He noted that a tweet from Steelman in December appeared to imply that Brunner "had no spine."
Campaigns often shift to a more aggressive message when they find something in their polling that they want to change or emphasize. It could be trying to prevent an opponent from gaining ground. Whatever prompted Brunner's new tactics, the playing field just got tougher.
"This is like a Jerry Springer show," said Jeff Roe, a Kansas City-based national Republican strategist. "It was only a matter of time before someone threw a chair."