Rep. Todd Akin, who played up his tea party credentials and conservative appeal, broke out from a three-way Missouri Republican primary on Tuesday to earn the right to take on Democratic Sen. Claire McCaskill, setting up one of the most closely watched Senate races of 2012.

Akin won a contest defined by which candidate was the most conservative. In doing so, he beat out Sarah Palin's candidate of choice, former state treasurer Sarah Steelman, and John Brunner, a businessman who poured more than $7.5 million of his own money into the race.

In Michigan, meanwhile, Republicans selected former Rep. Pete Hoekstra to oppose Democratic Sen. Debbie Stabenow in November. Democratic Rep. John Conyers staved off a primary challenge in a slightly redrawn district to advance to November's election, when he will be strongly favored to win a 25th consecutive term in Congress. Rep. Gary Peters defeated Rep. Hansen Clarke in a member versus member Democratic primary also brought on by congressional redistricting.

In another closely watched Missouri race, Rep. William Lacy Clay defeated Rep. Russ Carnahan in a showdown of two of Missouri's most prominent Democratic families. The two were also drawn together because of congressional redistricting.

Amid primary elections in four states Tuesday, Missouri's Republican Senate primary figured to have the most national significance: The GOP needs to net four seats from Democrats to gain control of the Senate and Republicans viewed McCaskill as among their top targets this year.

In a sign of the race to come, the candidates traded accusations of being a party extremist.

"The choice is clear in November," Akin said in a victory speech. "The big-spending, budget-busting, job-killing liberal or the less-spending, balanced-budget, job-creating conservative."

In an interview with The Associated Press, McCaskill said, "I don't know that Missouri voters will ever have more of a contrast." She added: "The issue here is not whether you can label him a conservative, but whether or not he is on the fringe — a very extreme candidate. I believe he is."

Akin, 65, who drew the backing of former GOP presidential candidate Mike Huckabee, has billed himself as a tea party supporter and has a strong conservative voting record in Congress. A former Army officer, engineer and state lawmaker, Akin first won election to the U.S. House in 2000 after narrowly winning a five-person Republican primary.

Polls indicated that any of the three GOP contenders would stand a good chance of defeating McCaskill. She took the unusual step of airing television ads targeting all three, a tactic that reflected the uncertain nature of the GOP primary.

In Michigan, Hoekstra enters his race against Stabenow as the underdog. Stabenow, the chairwoman of the Senate's agriculture committee, is seeking a third term and has enjoyed a steady lead in polls.

On the Democratic side in Michigan, Conyers defeated state Sen. Glenn Anderson. Conyers, who has had few serious primary challengers since first winning election in 1965, is expected to win the seat easily. Republican Harry Sawicki ran unopposed in the GOP primary.

In Washington state, Democrat Suzan DelBene advanced to take on Republican John Koster in the newly drawn 1st Congressional District. The seat is open after Democratic Rep. Jay Inslee resigned to run for governor. Inslee and Republican Attorney General Rob McKenna both advanced in the state's gubernatorial primary, setting the election for November.

The Kansas primary was defined by a fight between the state GOP's conservative wing and its more moderate elements. Conservative Republican challengers had unseated six GOP moderates in the Kansas Senate and led two others, improving their chances of reshaping the Legislature and ending a check on the political right's agenda.