By Kevin Murphy
KANSAS CITY, Missouri (Reuters) - Three Missouri Republicans are in a political free-for-all for the party nomination on Tuesday to challenge U.S. Senator Claire McCaskill, who political analysts consider the most vulnerable Democratic incumbent in the country this November.
The U.S. Senate primary race in Missouri is drawing the most national attention as voters in three other states - Michigan, Washington and Kansas - also go to the polls on Tuesday.
St. Louis businessman John Brunner held a narrow poll lead over conservative Congressman Todd Akin and former state senator Sarah Steelman as the tight Missouri race hurtled toward a close. Akin and Steelman were within striking distance and many voters were undecided.
"With the high undecided vote and the frenzy of the final week, its anybodys race," said Steve Glorioso, a political consultant who has worked on past McCaskill campaigns, but not yet this year.
A recent poll also showed that any one of the three challengers could knock off McCaskill in November, good news for Republicans in their effort to pick up at least four seats in the U.S. Senate and take the majority.
McCaskill is struggling in a state that has gone increasingly Republican after being a bellwether in presidential elections for a century. McCaskills support of Democratic President Barack Obamas healthcare reform has hurt her in the state, Glorioso said.
Brunner, who has never run for political office, is another in a string of insurgent Republican candidates who have shown they can upset the party establishment in Senate races this year. Brunner has bankrolled his own campaign and led most of the way.
Insurgents in Indiana, Nebraska and Texas have upset traditional Republicans, and others are running strong races in Arizona and Wisconsin.
The last poll before the Missouri vote published on Sunday by Democratic firm Public Policy Polling, showed Brunner with 35 percent, Akin with 30 and Steelman with 25 percent. One-in-ten voters were undecided.
In Michigan, another incumbent Democratic U.S. senator, Debbie Stabenow, is doing much better and has been comfortably ahead of any Republican challenger in recent polls.
The best known Republican in the Michigan race, former Congressman Peter Hoekstra, has been prone to gaffes that have hampered his campaign. Hoekstras first campaign ad this year was criticized as racist because it featured an Asian woman on a bicycle speaking broken English in an attempt to accuse Stabenow of selling out U.S. interests to China.
Hoekstra was leading his Tea Party movement-backed opponent Clark Durant in final polls before the voting.
Washington states primary advances the top two candidates in the vote count to the November election rather than holding separate Republican and Democratic primary elections.
Incumbent Democratic U.S. Senator Maria Cantwell is expected to cruise into the top two for the November election over a crowded but little-known field of challengers.
In Kansas, there is no U.S. Senate election this year and all four incumbent members of Congress are Republicans expected to sail through the primary.
(Writing and additional reporting by Greg McCune; Editing by Vicki Allen)