Republican Sen. Richard Lugar, the face of Indiana politics for nearly four decades, is battling for political survival against a tea party-backed GOP challenger in the state's primary, State Treasurer Richard Mourdock.
It is the highest-profile contest in four states with voting Tuesday.
Indiana's primary election is underway. The polls opened up at 6 a.m. for voters are casting their ballots.
Vanessa McClure from FOX Chicago's sister station in Indianapolis said early voting numbers are down in Marion County.
Only 9,000 submitted their ballots before the election this year. In Indiana's 2008 presidential primary, early voters totaled at 17,000.
The Marion County Clerk said the numbers are down because the GOP presidential candidate is already locked up.
She also said the race to watch in Indiana Tuesday is the race for 2nd district U.S. Senate seat. Based on Tuesday's election results, either Lugar or Mourdock will go on to face Democrat Rep. Joe Donnelly in the general election.
Mitt Romney is expected to coast to victory in primaries in Indiana, West Virginia and North Carolina as he continues his quest for the GOP presidential nomination.
(The Associated Press contributed to this report.)