INDIANAPOLIS (AP) — Democratic voters in Indiana were expected to head to the polls Tuesday in record numbers to cast ballots in a tight presidential primary race between Sens. Hillary Rodham Clinton and Barack Obama.

That influx of voters would affect congressional races across the state, political analysts said, likely helping challengers in Democratic primaries while hurting those hoping to upset Republicans.

All nine of Indiana's U.S. House members were on Tuesday's primary ballot, but four were running unopposed.

The toughest races were in the 7th District — where new Democratic Rep. Andre Carson faced seven challengers in his effort to retain the seat he won in a March special election — and in the 5th District, where longtime Republican Rep. Dan Burton faced what many saw as the strongest opponent of his 25-year career.

Carson hoped his brief experience in Washington set him apart from Democratic challengers, including former state Health Commissioner Woody Myers and state Reps. Carolene Mays and David Orentlicher.

But a high voter turnout could hurt Carson, said Robert Dion, a professor of American politics at the University of Evansville. The pool of voters in the urban district, which includes Indianapolis, would be much larger than the party loyalists who might favor Carson and appreciate the work done by his grandmother, Rep. Julia Carson, who died in December after 11 years in Congress.

Some voters drawn to the Clinton-Obama race knew little about the 7th District candidates. Media exposure — and the money it takes to pay for it — was important to win over those voters, Dion said.

"They go by what they've heard, and what they've heard is Woody Myers on TV," Dion said.

Myers has sunk more than $1.6 million of his own money on his campaign to help pay for advertising.

"We will see on May 6 how this investment pays off," Myers said last month. "I'm willing to do what it takes to get the job done."

The winner was likely to face state Rep. Jon Elrod — who lost to Carson in the March special election — although Cat Ping and Larry Shouse also sought the Republican nomination.

In the 5th District, emergency room physician John McGoff hoped to unseat Burton, the state's longest serving congressman.

Burton came under fire last year amid reports that he missed 19 House votes during a trip to California for a charity golf tournament. Burton has said the golf trip was a mistake and that House votes had typically not been held during that week in recent years.

McGoff's campaign played up Burton's travels with a Web site — wheresdanburton.com — that featured photos of the congressman's head on cartoon drawings.

"People are really ready for a change here," McGoff has said. "People are paying attention."

Getting out the GOP vote was key for McGoff. A low Republican turnout, which was possible because Sen. John McCain already had locked up the party's presidential nomination, likely would favor Burton as the party faithful support their incumbent.

Even if voters were unhappy and McGoff won a substantial portion of the vote, it would take a lot to actually defeat Burton in a primary, Dion said.

"He may get a slap in the face, but I'd be surprised if he lost," Dion said.

Three other incumbents — Reps. Mark Souder, Steve Buyer and Baron Hill — also faced primary challenges but were expected to breeze through the election. That would set up a familiar race in November in southern Indiana's 9th District, where Hill, a Democrat, is expected to face former Republican Rep. Mike Sodrel for a fourth consecutive time.

Indiana's other incumbents — Reps. Peter Visclosky, Joe Donnelly, Brad Ellsworth and Mike Pence — were unopposed in the primary.