(CNN) -- Voters from the Great Lakes to the Plains head to the polls Tuesday with primaries in Kansas, Michigan, and Missouri. In the spotlight are a competitive Republican Senate contest and gubernatorial primaries, as well as a key provision in the new federal health care reform law.
The GOP Senate battle is the marquee race in Kansas, as two congressmen fight to replace fellow Republican Sen. Sam Brownback, who is running for governor rather than for re-election.
According to the most recent polls, Rep. Jerry Moran heads into primary day with a lead over Rep. Todd Tiahrt.
"It's been a classic GOP primary, with each congressman claiming to be the true conservative. Moran has tried to be the more fiscal hawk and Tiahrt is probably closest to conservatives on social issues," says Nathan Gonzales, political editor for the nonpartisan Rothenberg Political Report.
Two other Republicans are also on the ballot. While five candidates are on the ballot in the Democratic primary, whoever captures the GOP contest will be considered the overwhelming favorite in the general election.
"The winner of the Republican primary essentially becomes the winner in November because Kansas is a state where Republicans dominate in federal races," adds Gonzales.
Expect competitive GOP primary contests to replace Moran in the state's 1st Congressional District and to replace Tiahrt in the 4th Congressional District. Nine Republicans and two Democrats are on the ballot in the 3rd Congressional District, hoping to succeed retiring Democratic Rep. Dennis Moore. One of the two Democrats is Moore's wife Stephene.
Brownback, who made a bid for the White House in 2008, is favored to win the GOP gubernatorial nomination, as well as the general election in November.
In Michigan, the state with the second highest unemployment level in the country, the contest for the GOP gubernatorial nomination is the headliner. The winner will start the general election with an advantage.
Recent state surveys indicate that businessman and former Gateway CEO Rick Snyder, Rep. Peter Hoekstra, the senior Republican on the House Intelligence Committee, and Attorney General Mike Cox are in a three-way battle, with Oakland County Sheriff Mike Bouchard in fourth place in a five-candidate field.
State Rep. Andy Dillon and Lansing Mayor Virg Bernero are facing off for the Democratic nomination. Two-term Democratic Gov. Jennifer Granholm is term-limited.
Michigan's economy continues to sputter and with a sitting Democratic president and an outgoing Democratic governor, "it looks like Michiganders might be willing to let Republicans have a shot at governing," says Gonzales.
There are also competitive Republican primaries for the 1st, 2nd and 3rd Congressional Districts. The GOP is hoping to score a pickup in November in the 1st Congressional District, where longtime Democratic Rep. Bart Stupak announced in April that he would not run for re-election.
Democratic Rep. Carolyn Cheeks Kilpatrick faces five opponents in her party's primary for the 13th Congressional District. The drama involving her son, former Detroit Mayor Kwame Kilpatrick, who is serving time for a parole violation, will not help her case.
There shouldn't be as much drama in Missouri, where the fight is to succeed retiring Republican Sen. Kit Bond.
Missouri Secretary of State Robin Carnahan, who hails from one of the state's leading political dynasties, is expected to easily win the Democratic primary. Rep. Roy Blunt, the former House Majority Whip, is expected to win in a nine-candidate Republican field. His only competition appears to be coming from Chuck Purgason, who enjoys the backing of many Tea Party activists.
One ballot initiative getting attention Tuesday is Missouri's Proposition C. It's a test of support for part of the new federal health care law, as voters decide if Missouri residents should be allowed to opt out of mandatory health insurance.
A recent Mason-Dixon poll conducted for the St. Louis Post-Dispatch and KMOV-TV indicated extremely strong Republican support for the proposition, with Democrats opposing the idea by a 48-27 percent margin.
Mason-Dixon pollster Brad Coker told the Post-Dispatch that the proposition is likely to pass, because there will be heavier Republican turnout for the primary. But if voters say yes, a court challenge is expected. Some states have already passed similar legislation, but this will be the first test at the ballot box.
CNN's Steve Brusk contributed to this report.