In a statement, Obama praised his former Senate colleague as someone "who was often willing to reach across the aisle and get things done."
The Mourdock vs. Donnelly match up could develop into a hotly contested race with the potential to affect the White House contest.
Obama carried Indiana in 2008, partly because of his ties to the populous northwestern part of the state neighboring his hometown of Chicago. Democrats acknowledge it will be difficult to win Indiana again this year. Still, the state could become more hospitable to Obama if the Democrats, believing they have a better chance with Lugar out of the race, spend heavily to compete against Mourdock. The state now is on the Obama team's watch list.
Elsewhere, North Carolina voters moved in the opposite direction from a string of states — Democratic-leaning places such as New York and Vermont as well as conservative Iowa — where same-sex marriage is now legal. Six states and Washington, D.C., now recognize gay unions.
North Carolina law already bans gay marriage, but the amendment on the state ballot effectively slammed that door. With 74 percent of precincts reporting, more than 60 percent had voted to strengthen the ban, while just under 40 percent had opposed it.
In the days before the North Carolina vote, two top administration officials — Vice President Joe Biden and Education Secretary Arne Duncan — expressed support for gay marriage. Obama supports most gay rights but has stopped short of backing gay marriage.
The Biden and Duncan comments sent the White House into damage-control mode as gay rights advocates pressed for him to publicly support of same-sex unions before November. Aides also tried to use the focus on the issue to criticize Romney's equivocations on gay rights over the years.
Obama's campaign said Tuesday that the president was "disappointed" with the state's amendment. Obama's spokesman for North Carolina, Cameron French, called the measure "divisive and discriminatory."
Romney, in turn, emphasized his position that marriage should be solely between one man and one woman. He has said he supports a federal constitutional amendment banning same-sex marriage.
In Wisconsin, voters chose Barrett — one of four Democrats on the ballot — to challenge Walker in the June 5 recall election.
Union rights are dominating the recall.
Walker effectively ended collective bargaining rights for most state workers and since then has emerged as a national conservative hero. The recall effort, mounted by opponents of his actions, has dominated the state political landscape, even overshadowing Romney's primary victory there that essentially ended the nomination fight.
Now the presumptive nominee, Romney had no serious opposition in Indiana, West Virginia and North Carolina on Tuesday.
Former Pennsylvania Sen. Rick Santorum, who gave Romney a tepid endorsement Monday night via email, and former House Speaker Newt Gingrich have dropped out of the race. Texas Rep. Ron Paul is still contesting the nomination, but he lags far behind in the delegate count.
LoBianco reported from Indianapolis. Associated Press writer Donna Cassata in Washington contributed to this report.
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