Mitt Romney, who has been a front-runner or near front-runner since last summer, is the apparent party nominee heading into the last round of primaries before the Republican National Convention in Tampa, Fla., in late August.
Of the candidates who began the primary-caucus season in Iowa, only U.S. Rep. Ron Paul of Texas remains an active challenger.
CBS News' estimated delegate count indicates Romney has 865 delegates, swamping Paul, who has 78. But the Paul campaign has been active in state party caucuses across the country, picking up delegates.
Indiana, North Carolina and West Virginia kick off the last round of primaries.
While it doesn't have a primary, Wisconsin polls are open Tuesday for the selection of a Democratic candidate to challenge Gov. Scott Walker in a June 5 recall election. The contenders are Milwaukee Mayor Tom Barrett, former Dane County Executive Kathleen Falk, Secretary of State Doug La Follette and state Sen. Kathleen Vinehout.
Veteran Sen. Dick Lugar of Indiana faces a tough primary challenge in state Treasurer Richard Mourdock, who has been embraced by Tea Party activists and others who question Lugar's conservative cred and/or think he's out of touch because he's been in the Senate too long.
The contest is the first time Lugar has faced a primary challenge since his election to the Senate in 1976.
It was bad news-good news for Lugar in the weeks leading to Tuesday's primary. Less than two weeks ago, the American Action Network, a major super-PAC backing Lugar, announced it was ending its involvement in the race, Yahoo! reported.
But Lugar picked up the endorsement of Gov. Mitch Daniels, who said on a recent edition of NBC's "Meet the Press" that Lugar was his role model.
"Folks in Indiana know that I am for him and that I admire him and think if he wants another term he ought to have one," Daniels said.
The Evansville Courier and Press reported independent groups spent more than $3.9 million in Indiana, paying for television, radio, Internet and direct mail ads in hopes of swaying the outcome of Tuesday's Republican Senate primary.
Why the interest?
"In Indiana and elsewhere across the country, there's a mini-civil war taking place within the GOP. Lugar and Mourdock represent both sides in that battle," Robert Schmuhl, an American studies professor at Notre Dame University, told the Courier and Press.
One group that has drawn ire for its involvement is the Young Guns Network, which is attached to the Young Guns group initiated by House Majority Leader Eric Cantor of Virginia and run by a former Cantor aide.
So far, the group has spent $208,000 in literature encouraging Democrats and independents to vote in the Republican primary and to support Lugar.
"Regrettably, Eric Cantor's actions confirm the worst of what grassroots conservatives dislike about a Washington Republican leadership that is more interested in protecting its own than in promoting conservative principles and candidates," said Barney Keller, a spokesman for the Club for Growth, which backs Mourdock.
"I'd thought the Young Guns were supposed to speak for the younger, more conservative and reformist wing of the GOP," Weekly Standard editor Bill Kristol said in an e-mail to the Daily Caller. "Who knew they'd be spending money trying to get Democrats to vote in a Republican primary for a moderate who thinks he's entitled to serve in the Senate until he's 86?"
North Carolinians will vote Tuesday on who will battle to succeed Gov. Beverly Perdue, a Democrat who isn't seeking a second term.
Voters also will decide on a Legislative proposal that would define marriage as a union between one man and one woman, and ban civil unions and domestic partnerships. Recent Public Policy Polling results indicated 55 percent of voters said they planned to vote yes while 41 percent said they were opposed.
In the governor's race, Lt. Gov. Walter Dalton pushed ahead of one-time U.S. Rep. Bob Etheridge, a PPP survey indicated, with state Rep. Bill Faison was well back at 5 percent.
On the Republican side, the presumptive nominee is former Charlotte Mayor Pat McCrory, who lost to Perdue in 2008. McCrory had a 66 percent favorable opinion among GOP voters in a poll late in April while the remaining candidates had less than 5 percent.
McCrory has a who' who list of fundraisers, including former Florida Gov. Jeb Bush, New Jersey Gov. Chris Christie, Louisiana Gov. Bobby Jindal and South Carolina Gov. Nikki Haley, Stateline.org said.
In a statewide race, some political observers said one state Senate race may require a runoff to determine a winner, The Fayetteville Observer said
Six people are on the ballot in the Democratic primary for an open seat. Incumbent Eric Mansfield is campaigning to become lieutenant governor this year.
No Republicans or libertarians are seeking the seat.
Under state law, one of the six contenders must win at least 40 percent of the vote to avoid a runoff later this summer. The Observer said state law requires a candidate to win a "substantial plurality" of the primary vote and set 40 percent as the threshold.
While North Carolina already bans same-sex marriage, the state is the only one in the Southeast not to change its constitution to bar gay unions, Yahoo! reported. The Republican-led state Legislature voted to put the matter before the voters this year.
West Virginians will be selecting nominees to run for two open seats on the state's Supreme Court Tuesday.
Six Democrats are on the ballot and the top two will face two Republicans who are running unopposed.
On the Democratic ballot are incumbent Justice Robin Jean Davis of Charleston, former West Virginia State Bar President Letitia Chafin of Charleston, Circuit Court Judge James Rowe of Lewisburg, Circuit Judge J.D. Beane of Parkersburg, Louis Palmer of Pocatalico and H. John Rogers of New Martinsville.
Advancing to the general election ballot are Republicans Allen Loughry of Charleston and John Yoder of Harpers Ferry.
In West Virginia's 2nd Congressional District, three Democrats are vying to take on incumbent Shelley Moore Capito, who faces two challengers in the Republican primary.
As of Thursday, the Secretary of State's office said it had received 37,518 early-voting and absentee ballots.