Sen. Claire McCaskill (D-Mo.) has problems, but Missouri Republicans aren’t sold on the field they have to run against her, either. (AP Photo/Manuel Balce Ceneta) (Manuel Balce Ceneta/AP)

The state’s swing nature coupled with McCaskill’s narrow win in 2006 made the race a ripe opportunity, Republican strategists argued, but few big names seemed ready to step up to the plate.

McCaskill’s problems over her private plane — and, more specifically, the unpaid back taxes on it — have made her even more vulnerable.

But will the Republican field expand? Or is it set?

Missouri Republican Party executive director Lloyd Smith said Tuesday that recent events have only increased interest in running against her.

“Let’s just say there is a heightened awareness,” Smith said. Though he declined to name names, he added that candidates “have shown an increased interest in at least thinking about running for this office.”

The question is – who?

Former state Treasurer Sarah Steelman and former congressional candidate Ed Martin are already in the race, while Rep. Todd Akin and former Ambassador to Luxembourg Ann Wagner,who ran for the chairmanship of the Republican National Committee earlier this year, are both weighing their options.

Depending on who you ask, none of the above names is the party’s ideal candidate for the seat. And given McCaskill’s problems, it would seem some other Republicans might see a golden opportunity to get to the Senate..

But former Sen. Jim Talent and Reps. Sam Graves and Jo Ann Emerson have already said they won’t run, Lt. Gov. Peter Kinder is likely to run for governor, and former Gov. Matt Blunt, who has worked in Washington since leaving office, has never been seriously mentioned for the post. (Plus his father, Roy, is the state’s other senator. Matt Blunt told The Fix he doesn’t “have any plans” to run — which isn’t a categorical denial but seems genuine.)

Beyond those names, there’s not a whole lot out there — at least that’s we’re seeing right now. (There’s always the possibility of a wealthy business person — ala Ron Johnson in Wisconsin — emerging but none has yet.)

If the GOP field then is some combination of Steelman, Martin, Akin and Wagner, who is the favorite?

“(McCaskill) is burning on the stake right now,” said one unaligned Missouri Republican consultant. “I think our bench is just too weak, but it has to push Akin further in.”

Akin, a backbencher from the St. Louis area, would come into the race with plenty of conservative bona fides, which has caused some to label him the early favorite. But he’s kept a relatively low profile, has not faced a tough race since winning the primary for an open House seat in 2000 and is an earmarker, having ranked in the top 100 of House earmarkers as recently as 2008. That’s bad news in a Republican primary where voters soundly rejected the business-as-usual sort of candidates in 2010.

Others see Steelman as the early favorite. She nearly upset the GOP establishment candidate in the 2008 governor’s primary and has built bridges with the tea party –- including winning some kind words from Sen. Jim DeMint (R-S.C.). But while she might be a strong primary candidate, not everyone is sold that Steelman would be Republicans’ strongest nominee for the general election.

Martin lost a 2010 challenge to Rep. Russ Carnahan (D) by two points. It was a solid effort in a tough district — Carnahan’s 3rd district went 60 percent for President Obama in 2008 — but few see Martin as senatorial timber just yet.

Wagner can raise money, but she carries very little name recognition and is seen by some as a more likely House candidate – potentially for Akin’s seat if he runs.

If recent history is any guide, not finding the perfect candidate against McCaskill may not matter.

The last three elections are filled with candidates that weren’t initially considered major recruits but turned into solid campaigners who won. That list includes now-Sens. Jon Tester (D-Mont.), Jim Webb (D-Va.), Jeff Merkley (D-Ore.), Kay Hagan (D-N.C.), John Boozman (R-Ark.), Chris Coons (D-Del.), Ron Johnson (R-Wis.) and Marco Rubio (R-Fla.).

Almost all of these candidates, it should be noted, were running in very favorable environments and often against very damaged opponents, factors that aided them along to victory

McCaskill’s damage looks bad now, but as we noted Tuesday, 20 months is a lifetime in politics.

Republicans should have a shot either way in Missouri — a state that defines “swing state” and narrowly went for Sen. John McCain (R-Ariz.) in 2008. The question is how good a shot it will be.

For now, both McCaskill and the GOP field looking to run against her have something to prove.