(Adds details on races in Michigan and Kansas)
By Carey Gillam
KANSAS CITY, Mo., Aug 5 (Reuters) - Kansas voters will get to weigh in Tuesday on a Republican food fight in a congressional primary and decide a close contest between a veteran U.S. senator and a physician who has family ties to President Barack Obama.
As four U.S. states hold primaries on Tuesday, one Kansas race drawing national attention is for the state's 4th Congressional District, where food labeling has become an issue in Republican Todd Tiarht's challenge to incumbent U.S. Representative Mike Pompeo.
Pompeo, backed by powerful food and agriculture companies, recently introduced legislation to nullify state efforts to require labeling on foods made from genetically modified crops.
Laws mandating such labeling are being pursued in several states, and Tiahrt has found support in colliding with Pompeo over the issue.
"Tiahrt appears to be running a 'scorched earth' policy of attacking many traditional Republican donors and supporters in an effort to paint himself as a populist," said Michael Smith, political science professor at Emporia State University.
Pompeo succeeded Tiahrt in the House of Representatives when the latter made an unsuccessful U.S. Senate bid in 2010.
Also in Kansas, Republican Senator Pat Roberts finds his 47-year career in Congress in jeopardy as polls show conservative "Tea Party" challenger Milton Wolf, a doctor who says he wants to "save the Republic," closing in.
Wolf has acknowledged a distant family tie to Obama but has built his campaign on promises to repeal many of the Democratic president's policies. In an interview with CNN, Wolf promised "the mother of all family feuds to save America," if elected.
The other U.S. states holding primaries are Missouri, Michigan and Washington.
Voters in Missouri will have a say on whether John "Jay" Ashcroft, whose father, John Ashcroft, was Missouri governor, U.S. attorney general and a U.S. senator, moves forward in his bid for an open seat in the state senate.
Five of Missouri's U.S. Representatives face primary challengers but are expected to hold their seats going into November's general election.
In Michigan, the Republican primary features incumbent U.S. Representative Justin Amash and challenger Brian Ellis. Amash is a Tea Party favorite and member of a rebel group of House conservatives known for their resistance to compromise, while Ellis is the head of an investment firm who bills himself as "West Michigan Nice" for his collaborative style.
Also in Michigan, U.S. Representative Kerry Bentivolio, known as a reindeer farmer and Santa Claus impersonator, looks poised to lose to challenger Dave Trott, which would make Bentivolio the third incumbent Repulican congressman to lose in a primary so far this year.
In Washington state, retired Microsoft engineer Pedro Celis is taking on businessman and perennial candidate "Mike The Mover," formerly known as Michael Shanks, to represent the 1st Congressional District. Celis, former chairman of the Republican National Hispanic Assembly, is expected to win the primary easily to face off in November against the incumbent Democrat, U.S. Representative Suzan DelBene. (Editing by Eric Walsh and Doina Chiacu)