Brunner is an established donor to GOP candidates, including to Rep. Todd Akin (Mo.), one of the candidates he now faces in a Republican primary. He rejected the notion that his previous support for Akin would pose a problem and said he would draw the same distinctions between Akin and himself that he would draw between Republicans and centrists like McCaskill.

Brunner’s other primary opponent, former state Treasurer Sarah Steelman, greeted Brunner’s candidacy Monday in a Twitter post with the hashtag “#bringit” appended.

“Welcome to the race! The water is fine,” Steelman wrote.

{mosads}A poll released Sept. 20 by Democratic firm Public Policy Polling showed Steelman in the lead with 40 percent, followed by Akin at 29. Brunner, who had not yet entered the race, took 6 percent.

The winner of the primary will take on McCaskill, a first-term senator whom Republicans consider extremely vulnerable and are actively targeting, and whom Democrats are already aggressively working to buttress.

Even before Brunner entered the race Monday, Democrats had filed a complaint against him with the Federal Election Commission, alleging Brunner violated the rules by spending too much money in support of his candidacy without actually declaring or registering his candidacy, which would have triggered financial reporting requirements.

“If Brunner’s operating his campaign like this, we know we can expect more big-money, backroom deals from him if he’s elected, and that’s the last thing we need more of in Washington,” said Caitlin Legacki, a spokeswoman for the Missouri Democratic Party.

McCaskill has been weighed down by a pallid economy and high unemployment that the Democratic leadership in the White House and Senate has been unable to upend. She has also been wounded by appearances at town-hall meetings where she was unable to quiet the protests of Tea Party members, and by the fallout of a blunder over taxes she failed to pay on a private plane. (She later paid the back taxes owed.)

An additional blow to McCaskill, a centrist whom Republicans argue is too liberal for the state, has been Obama’s dragging poll numbers in Missouri, where more than half of voters disapprove of his job performance. Although a crucial swing state, Obama lost Missouri in 2008 by fewer than 4,000 votes.

McCaskill holds a small lead over all the GOP candidates — within the margin of error for Steelman and Akin — according to a PPP poll released Sept. 13. On the plus side for McCaskill, her 43 percent approval rating has not significantly decreased in recent months. On the negative side, that number puts her second from the bottom on the list of Democratic senators whose approval ratings the firm has polled — only Sen. Ben Nelson (D-Neb.) polls worse.

Brunner said McCaskill’s biggest weakness is the same faced by incumbents all across the country: She is perceived as a career politician, and not one with the real-life experience needed during tough times to develop effective solutions.

“If you need quadruple-bypass, would you go to the dean of the medical school, or would you go to the guy who’s performed 2,000 operations?” he said.