WASHINGTON (AP) -- Six-term veteran Indiana Sen. Richard Lugar lost a bitter challenge from the right flank of his own Republican Party Tuesday night, his nearly four-decade career in the Senate ended by a tea party-backed GOP foe.
"I have no regrets about running for re-election. Even if doing so can be a very daunting task," the 80-year-old Lugar said as he conceded to state Treasurer Richard Mourdock.
Conservatives reached for another victory Tuesday as North Carolina voters weighed a gay-marriage ban, and Democrats were picking a nominee to challenge Republican Wisconsin Gov. Scott Walker in a June recall election, contests that overshadowed Mitt Romney's unstoppable progress toward the GOP presidential nomination.
Romney won the GOP presidential primaries in Indiana, North Carolina and West Virginia, drawing close to the 1,144 delegates he needs to clinch the nomination.
Even Romney, the former Massachusetts governor, was essentially ignoring primaries that were likely to hand him 100 or so delegates of the 288 he still needed heading into Tuesday's contests. He spent the day campaigning in Michigan, where he castigated President Barack Obama as an "old-school liberal" whose policies would take the country backward.
The outcomes of Tuesday's far-flung voting were certain to give clues about the state of the electorate -- and highlight the political minefields facing both Republican and Democratic candidates -- six months before the general election.
In the biggest race of the night, Lugar lost to tea-party backed state Treasurer Mourdock, who will face Democratic Rep. Joe Donnelly in the November general election. Republicans need to gain four seats to take control of the U.S. Senate, and a Lugar loss "gives Democrats a pickup opportunity," said Sen. Charles Schumer, D-N.Y.
Lugar ruled out running as an independent.
"This is it," he said with voting still under way.
Playing out in a conservative state, the race illustrated the electorate's animosity toward many incumbents and anyone with deep ties to Washington. That was clear when Lugar, who hasn't faced questions about his residency in decades, found himself on the defensive over whether he lived in Indiana or northern Virginia. Lugar also was cast as too moderate for the conservative GOP in Indiana, and he took heat for his work with Democrats on issues such as nuclear nonproliferation, underscoring deep polarization in the country as well as a split in the GOP between the establishment wing and the insurgent tea party.
In a statement, Obama praised his former Senate colleague as someone "who was often willing to reach across the aisle and get things done."
On Capitol Hill, Republicans braced for Lugar's loss throughout the day.
"It says if you're an incumbent, you better not lose touch with home," said South Carolina Republican Sen. Lindsey Graham.
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 weighed in on whether to pass a state constitutional amendment defining marriage as solely between a man and a woman. Should the gay marriage ban pass, the state would be moving in an 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 would effectively slam that door.
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 come out in 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.
Romney, in turn, emphasized his position that marriage should be solely between one man and one woman. He has said that he supports a federal constitutional amendment banning same-sex marriage.
In Wisconsin, voters were deciding whether to give Milwaukee Mayor Tom Barrett -- one of four Democrats on the ballot -- a rematch against Gov. Walker in the June 5 recall election or whether to back one of Barrett's fellow Democrats.
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 e-mail, 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.