Indiana's Republican voters will decide Tuesday which of five U.S. Senate candidates will represent the party in its fight to take the seat being vacated by Democrat Evan Bayh.

GOP leaders had high hopes that former Sen. Dan Coats, a senator for about eight years in the 1990s, would be a shoo-in for the nomination. National Republicans have backed him since he launched his campaign in February, shortly before Bayh made his surprise announcement that he wouldn't seek a third term.

But Coats' attempt at a political comeback hasn't been as smooth as some expected.

Some voters have been put off by lingering questions about his ties to Washington, D.C., and his work as a lobbyist. And two of his leading opponents — former Rep. John Hostettler and state Sen. Marlin Stutzman — have tried to portray themselves as more conservative.

"They've been playing who can get furthest to the right," said Brian Vargus, a political science professor at Indiana University-Purdue University in Indianapolis.

Analysts predict Coats could be hurt Tuesday if tea party voters and more conservative Republicans come to the polls in large numbers, though he'll be helped by the crowded field of GOP candidates that also includes financial adviser Don Bates Jr. and businessman Richard Behney.

The winner of Tuesday's race will face Democratic Rep. Brad Ellsworth in November. A conservative Democrat, the second-term congressman is to be formally nominated by the Democratic central committee May 15. He entered the race four days after Bayh announced his retirement.

Coats won a special election in 1990 to serve the remainder of Dan Quayle's term after Quayle became vice president in 1989. Coats' name was last on an Indiana ballot in 1992, when he made a successful bid for a full Senate term.

But he decided not to run for re-election in 1998, when Bayh made his first run for the seat. Coats has since been an ambassador to Germany under former President George W. Bush and worked as a lobbyist in Washington.

Although Democrats don't know who they'll be up against in November, they've already attacked Coats.

The chairman of the Indiana Democratic Party chided Coats for missing a deadline to file a personal financial disclosure report, saying he should know better because of his experience as a former senator. Once Coats filed the form, Democrats criticized him for being an "elite D.C. lobbyist" and questioned whether he would represent Indiana residents or special interest groups.