WASHINGTON - Six-term Indiana Sen. Richard G. Lugar was routed by the right flank of his Republican Party on Tuesday, and North Carolina voters decided overwhelmingly to strengthen their state's gay-marriage ban - a conservative show of enthusiasm and strength six months before the nation chooses between Democratic President Obama and Republican Mitt Romney.

Romney swept three primaries, moving ever closer to sealing his nomination.

"I have no regrets about running for reelection, even if doing so can be a very daunting task," Lugar, 80, said as he conceded to the tea party-backed GOP opponent who ended his nearly four-decade career in the Senate. Lugar's foe, Indiana Treasurer Richard Mourdock, had painted the Republican senator as too moderate for the state.

North Carolinians voted to amend their state constitution to define marriage as between one man and one woman, effectively outlawing gay unions.

The contests overshadowed Romney's continued progress toward the Republican presidential nomination. He won primaries in Indiana, North Carolina, and West Virginia, drawing close to the 1,144 delegates he needs to clinch the nomination. He was likely to win 100 or so delegates of the 288 he still needed.

In the biggest race of the night, Lugar lost to Mourdock, who will face Democratic Rep. Joe Donnelly in November.

Within minutes of Lugar's loss, Democrats were painting Mourdock as too extreme for the state.

Tea party groups were crowing about the win, and Mourdock urged supporters to donate to his general-election campaign, saying: "We left everything on the table to win the primary."

Republicans need to gain four seats to take control of the Senate, and a Lugar loss "gives Democrats a pickup opportunity," said Sen. Charles E. Schumer (D., N.Y.).

Earlier in the day, Lugar, 80, made clear he would stand by Tuesday's outcome, ruling out running as an independent.

"This is it," he said.

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 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."

The Mourdock-vs.-Donnelly matchup 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, but Democrats acknowledge it will be difficult to win Indiana this year.

In North Carolina, voters approved a constitutional amendment defining marriage solely as a union between a man and a woman.

State law already bans gay marriage, but an amendment strengthens the ban. The amendment also goes beyond state law by voiding other types of domestic unions from carrying legal status, which opponents warn could disrupt protection orders for unmarried couples.

In the days before the North Carolina vote, two top administration officials - Vice President 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 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.