Voters across four states head to the polls today to decide primaries, with the GOP Senate race in Missouri getting marquee billing.

The races worth watching are in Michigan, Missouri and Washington. Kansas is also holding primaries, but all four Congressional races are rated as Safe Republican and not competitive this fall.

There are two Member-vs.-Member races on tap today, and both feature a white Member versus a member of the Congressional Black Caucus. In both instances, however, one Member has a clear advantage over the other going into primary day. In Missouri, Rep. William Lacy Clay is expected to easily defeat Rep. Russ Carnahan in a Democratic battle of well-known Missouri political families. In Michigan, Rep. Gary Peters is the clear favorite over Rep. Hansen Clarke in a redrawn Detroit-based district.

Two other races of note in the Wolverine State are the Democratic primary where Rep. John Conyers is seeking a 25th term and the wacky race in the 11th district, where the GOP establishment is backing a write-in candidate.  

Polls in Missouri are open from 6 a.m. to 7 p.m. CST. Here are the races worth watching tonight:

Senate

There’s no prohibitive frontrunner in today’s very competitive and expensive Republican Senate primary in Missouri. The victor could easily be Rep. Todd Akin, businessman John Brunner or former state Treasurer Sarah Steelman.

The underdog in the Show-Me State, however, is clear: vulnerable Democratic Sen. Claire McCaskill, who will face the winner of the primary in November.

Brunner and Steelman, both partial self-funders, are seen as the stronger general election candidates by Democrats — and some Republicans, too. Akin is seen as a lackluster campaigner, burdened by his comfort with earmarking, among other weaknesses.

Still, in a recent nonpartisan public poll for St. Louis media outlets, all three GOP challengers led the first-term incumbent in horse-race matchups.

Brunner had an edge over his competitors in that poll and others, but not so much of an edge that he can be seen as the true frontrunner, Missouri insiders said.

“I would not be surprised with any of them winning,” said one top Missouri GOP consultant who has been looking at recent internal tracking polls. “This thing could be decided by less than 10,000 votes. It’s going to be tight as hell.”

Brunner, the former CEO of pharmaceutical and personal-care product manufacturer Vi-Jon Inc., has put millions of dollars of his own fortune into his campaign.

All that money has worked to raise his visibility in a state where he was completely unknown as recently as last year. Brunner allies believe their messaging has worked.

“At the end of the day, where Brunner cuts through is: not a politician, a businessperson; not a politician, not part of the problem,” a Brunner aide explained.

Akin and Steelman have had their own moments of cutting through in recent weeks. Akin, who has a base among social conservatives, got a real boost when former Arkansas Gov. Mike Huckabee (R) narrated a strong TV spot for him that aired in July.

Steelman, for her part, rolled out a potent endorsement from former Alaska Gov. Sarah Palin (R), who campaigned in Missouri with the former state treasurer in recent days.

McCaskill’s aides believe all three candidates have significant flaws. The campaign has been chomping at the bit to have a clear frontrunner with whom to make a contrast.

Her campaign even released three ads, each knocking one of the potential GOP nominees. The one that was most intriguing was a rather tepid “attack” against Akin for being too conservative. That appeared to be an effort to boost Akin in the primary, in hopes he will squeak out a victory.

McCaskill has spent more than $850,000 on TV and radio ads in the past three weeks, according to a Republican tracking the buys.

Whomever the winner is tonight, Democrats look forward to bringing up his or her negatives rather quickly.

“Up until this point, because of the outside advertising and because the primary has given the lackluster candidates cover, it has been a referendum on McCaskill,” said a Democratic operative familiar with the race. “But starting Wednesday, that dynamic changes.”

Indeed, McCaskill has been buffetted this cycle by millions of dollars in outside attack ads that have emphasized her connection to President Barack Obama, who is almost certain to lose the Show-Me state. If she is to have a real shot of coming back to the Senate next year, McCaskill and her allies will have to knock her opponent down quickly — and hard.

Watch for that effort to begin as soon as the GOP nominee is decided tonight.

House

1st District

This is a race Democrats in Washington, D.C., did not want. In redistricting, the home of Rep. Russ Carnahan, who is white, was drawn into the same urban St. Louis district as the home of Rep. William Lacy Clay, who is black. Carnahan’s current district was essentially dismantled in the redraw, which left him with an unenviable choice. He could run in the 1st, a safe Democratic majority-minority seat, against a fellow Democratic Member; he could run in the newly-configured open 2nd district, which is GOP-leaning; or he could retire or run for another office.

He chose the first option, to the considerable chagrin of top Democrats such as Rep. Steve Israel (N.Y.), chairman of the Democratic Congressional Campaign Committee. Israel and others had tried to persuade him to run in 2nd, where he would have had a shot at picking up a GOP seat.

The day of the primary, it seems like Carnahan probably didn’t make the right choice. Insiders see Clay poised for a solid victory. A poll by SurveyUSA in the field late last week found Clay leading a horse-race matchup of likely Democratic primary voters 56 percent to Carnahan’s 35 percent. Only a third of Carnahan’s current constituents were drawn into the newly configured 1st, which put him at a disadvantage from the start of his campaign.

While the Carnahan name is powerful and well-known in Missouri — members of his family have held most of the top elective offices in the state — the Clay name is probably more potent in the 1st district. Clay’s father served as a Member from 1969 through 2000 and was directly succeeded by his son. The Clay reign looks like it will continue for at least two more years.