Indiana does not register voters by party. That means voters can cross over at will, requesting a Republican ballot one election and a Democratic one the next.
Voters have been urged to cross lines before. In 2008, conservative talk-show host Rush Limbaugh urged Republicans to vote for Hillary Rodham Clinton over Barack Obama in the Democratic primary.
Leaders in both parties tell The Indianapolis Star (http://indy.st/JwSAby) it's rare for true party faithful to ask for the other party's ballot, even if the top races on the ballot aren't contested, as is the case with the Democratic races for president, governor and Senate in Indiana this year.
Indiana Democratic Party Chairman Dan Parker said Lugar has lost support among Democrats and independents by taking a right turn in his primary election fight against Mourdock.
"Dick Lugar's strategy to try to move as far right as he could kind of (cut off) that opportunity. By attacking the president and destroying his image of bipartisanship, he hurt his chances of getting Democrats to participate in the primary," Parker said.
Though it's more likely that most Democrats will sit out the primary or stick with their party, some say they've decided to back Lugar.
Nora Loechel, 44, of Indianapolis, said she usually votes Democratic but will vote in the GOP primary to support Lugar this time.
"Lugar's history of bipartisanship is critical right now," Loechel said. "We so badly need Congress to get along and work together, not put more people in whose main goal is to divide and conquer."
Lugar has urged everyone who has supported him over the years to vote for him.
His campaign said it isn't urging Democrats to pick up a GOP ballot this time but is seeking out independent voters who occasionally cast ballots in the Republican primary.
"That is the bigger tent that has been Dick Lugar's support," Lugar spokesman Andy Fisher said.
Information from: The Indianapolis Star, http://www.indystar.com