Incumbent lawmakers in Kansas, Michigan and Missouri sought to repel challenges from within their party as a busy month of primaries got underway Tuesday. Voters in Washington state, meanwhile, considered a replacement for a two-decade congressional Republican who is retiring.

In Kansas, three-term Republican Sen. Pat Roberts faced Milton Wolf, a radiologist and the distant cousin of President Obama. Wolf and his tea party allies cast the 78-year-old Roberts as a longtime fixture in Washington who barely lives in the state, and Roberts didn't help his cause when he told a radio interviewer last month, "Every time I get an opponent -- uh, I mean, every time I get a chance -- I'm home."

Roberts cast Wolf, 43, as unfit for elected office, questioning his judgment. Roberts' ads have focused on Wolf's posting in 2010 of X-ray images of patients' injuries on his Facebook page. Wolf acknowledged in February that he'd made the Facebook postings and has apologized. The postings included images of fatal gunshot wounds and comments intended to be humorous.

Jerome Desch, a 72-year-old retiree from Topeka, said he voted for Roberts because of the X-ray images.

"I think it shows your true colors," Desch said.

Voting at the same Topeka site, 69-year-old Rozella Rose, a business manager, said she chose Wolf over Roberts, in part because she believes Roberts has been in Congress too long.

"He's younger and he's more conservative," Rose said of Wolf.

If past primaries are any guide, voter turnout will be low and the elections will be decided by the most fervent members of each party.

In Michigan, a pair of establishment-minded Republicans leaned on outside groups -- and their own fortunes -- in bids to knock off incumbent House members who often side with the tea party.

Dave Trott is challenging first-term Rep. Kerry Bentivolio and has contributed almost $2.5 million of his own money to the race. Brian Ellis, an investment adviser who lent his campaign $1 million, is challenging Rep. Justin Amash, who was elected in 2010's tea party wave but has sparred with fellow Republicans since.

A third member of the state's congressional delegation, Rep. Dan Benishek, is expected to survive his primary challenge from tea partyer Alan Arcand in a northern Michigan district. The U.S. Chamber of Commerce spent $200,000 on ads for Benishek.

Voters are also set to nominate Republican Terri Lynn Land to face Democratic Rep. Gary Peters in the Senate race. Six-term Sen. Carl Levin, a Democrat, is not seeking re-election and Michigan's race is considered competitive.

Rep. John Dingell, a Dearborn Democrat who has been in Congress for a record 58 years, is retiring. His wife, Debbie, is expected to extend the Dingell dynasty beyond its already 81-year run.

In Missouri, five of the state's eight House members faced primary challenges from badly underfunded opponents. Together, the eight incumbents had almost $5 million in the bank. Their 38 challengers' campaigns had less than $100,000 combined.

Missouri voters also were considering a ballot issue that would increase sales taxes to pay for more than 800 transportation projects. The proposal would generate more than $540 million each year for the next decade.

In Washington state, voters considered 12 candidates vying to replace 10-term Rep. Doc Hastings, a Republican who is retiring. The two candidates who collect the most mail-in ballots advance to the general election, setting up what could be a Republican-versus-Republican contest in November in the heavily Republican district in central Washington.