WASHINGTON (AP) — All along, the Republicans have considered Democratic Sen. Claire McCaskill of Missouri one of their top targets in the Senate this year. They just haven't known who her GOP opponent would be.

Party voters decided on Tuesday among three contenders — Sarah Palin-backed Sarah Steelman, businessman John Brunner and Rep. Todd Akin, who was endorsed by former Arkansas Gov. Mike Huckabee — in the marquee contest of four states' primaries. No clear favorite emerged in advance of the Missouri balloting that was setting up one of November's most anticipated Senate contests.

All three Republicans cast themselves as the best conservative alternative to McCaskill, who is seeking re-election for the first time since winning her seat in 2006.

Other races in the four states with elections Tuesday included:

— A Republican primary in Michigan to determine who will take on Democratic Sen. Debbie Stabenow, the chairwoman of the Senate Agriculture Committee.

— Member- vs.-member primary contests in Michigan and Missouri, including a showdown between two of Missouri's most prominent Democratic families. Reps. Russ Carnahan and William Lacy Clay vied for a St. Louis-area seat in a race brought on by congressional redistricting.

— A Democratic congressional primary for an open seat in Washington state.

In addition, longtime Democratic Rep. John Conyers was on the ballot in Michigan, seeking to hold off a primary challenge in his district.

And in Kansas, Republican primaries could determine whether a conservative bloc would take control of the state Legislature.

The closely watched Missouri race between Steelman, Akin and Brunner promised the most national consequence. The GOP needs to net four seats from Democrats to gain control of the Senate in 2012.

The GOP hopefuls have lambasted McCaskill for what they say are her close ties to President Barack Obama and, in particular, her vote for Obama's signature health care law. The race took on a different dynamic in recent days as Palin and Huckabee — two of the GOP's most prominent faces — lent their backing to candidates.

Palin, in particular, has been dogged in her support of Steelman, a former state treasurer. Palin has appeared in television and radio ads and campaigned with Steelman at a series of events in the state. Steelman, 54, has said she hopes to capitalize on the momentum of Ted Cruz, the Republican nominee for the Senate in Texas. He rode strong tea party support — and a timely boost from Palin — to an upset victory in that state's Senate primary last week.

Akin, 65, has also billed himself as a tea party supporter and has a strong conservative voting record in Congress. But both Steelman and Brunner have sought to use that experience against him, portraying themselves as outsiders.

The 60-year-old Brunner is a former CEO and chairman of Vi-Jon Inc., a health care products manufacturer. He has spent more than $7.5 million of his own money to campaign for the seat. He stops short of calling himself a tea party candidate, but he has the backing of FreedomWorks, a national tea party group, and the conservative Club for Growth. Both groups endorsed Cruz in Texas.

Polls indicate that any of the three would stand a good chance of defeating McCaskill. She has taken the unusual step of airing television ads targeting all three, a tactic that reflects the uncertain nature of the GOP primary.

Republican voters in Michigan were picking between two candidates who cast themselves as conservatives. That contest pitted former Rep. Pete Hoekstra against Clark Durant, with the winner taking on Stabenow. She is running for her third term and has enjoyed a steady lead in polls.

On the Democratic side in Michigan, Conyers was seeking to hold off a primary challenge that was prompted, in part, by tweaks to his congressional district because of redistricting. In another race affected by redistricting, incumbent Democratic Reps. Hansen Clarke and Gary Peters were running against each other in a contest that will see one of them leave Congress next year.

In Washington, seven people were running for a seat representing the newly drawn 1st Congressional District. They included Democrats Suzan DelBene, Darcy Burner, state Sen. Steve Hobbs, Laura Ruderman and Darshan Rauniyar, Republican John Koster; and independent Larry Ishmael. Washington state votes by mail, so all of the state's 3.7 million voters received their ballots weeks ago. Voters had to have their ballots postmarked and in the mail by Tuesday or drop them off at special boxes around the state by 8 p.m. local time.

The Kansas primary was defined by a fight between the state GOP's conservative wing and its more moderate elements. Conservatives were hoping to use the primary to take decisive control of the state Senate, where moderate Republicans have slowed efforts to cut taxes, shrink government and pass more conservative social policies. A dozen moderate incumbent Republicans faced challenges from the right, including the Senate president.