6:07PM UPDATE: Santorum wins Republican presidential primary in Oklahoma.
6:05PM UPDATE: In early returns, Mitt Romney and Rick Santorum are nearly tied in Ohio.
6PM UPDATE: Rick Santorum has won the Republican presidential primary in Tennessee.
Former House Speaker Newt Gingrich countered with a home-field win in Georgia as the GOP contenders vied for the chance to take on Democratic President Barack Obama in November.
Romney also dueled Rick Santorum in Ohio, their second industrial-state showdown in as many weeks and the marquee matchup of the busiest night of the race.
Santorum and the fourth contender, Rep. Ron Paul, sought success elsewhere, with contests stretching across the country and more than 400 Republican National Convention delegates at stake.
Win or lose there, Romney said "I think we'll pick up a lot of delegates, and this is a process of gathering enough delegates to become the nominee and I think we're on track to have that happen."
There were primaries in Tennessee and Oklahoma as well as Virginia, Vermont, Ohio, Massachusetts and Georgia. Caucuses in North Dakota, Idaho and Alaska rounded out the calendar.
Gingrich's victory was his first since he captured the South Carolina primary on Jan. 21, and the former House speaker said it would propel him on yet another comeback in a race where he has faded badly over the past six weeks.
Obama, the man they hope to defeat in November, dismissed the almost-constant criticism of his foreign policy efforts and accused Republicans of "beating the drums of war" over Iran. "Those folks don't have a lot of responsibilities. They're not commander in chief," he said. Unopposed for the Democratic nomination to a second term, he stepped into the Republican race with a Super Tuesday news conference at the White House, then attended a $35,800-a-ticket fundraiser a few blocks from the White House.
He later attended a $35,800-per-ticket fundraiser a few blocks from the White House.
In Georgia, Gingrich was gaining 48 percent of the vote in early returns, with Santorum at 24 percent and Romney close behind at 22 percent.
In Virginia, Romney had 59 percent and Paul had 41 percent. Santorum and Gingrich failed to qualify for the ballot.
Romney led in Vermont with 39 percent of the vote in early returns, with Paul gaining 27 percent and Santorum 24 percent.
Ohio was the day's biggest prize in political significance, a heavily populated industrial state that tested Rick Santorum's ability to challenge Romney in a traditional fall battleground. Georgia, Newt Gingrich's home political field, outranked them all in the number of delegates at stake, with 76, a total that reflected a reliable Republican voting pattern as well as population.
Romney, the leader in the early delegate chase, flew home to Massachusetts to vote and said he hoped for a good home-state win. Overall, he added, "I think we'll pick up a lot of delegates, and this is a process of gathering enough delegates to become the nominee and I think we're on track to have that happen."
He also took issue with Obama, saying, "I think all of us are being pretty serious" about Iran and its possible attempt to develop nuclear weapons.
Gingrich, hoping for a victory in Georgia, effectively acknowledged he had scant Super Tuesday prospects elsewhere. Instead, he was pointing to primaries next week in Alabama and Mississippi, and he told an audience, "With your help, by the end of next week we could really be in a totally new race."
The polls show the president's chances for re-election have improved in recent months, as the economy has strengthened, unemployment has slowly declined and Republicans have ripped into one another in the most tumultuous nominating campaign the party has endured since 1976.
No matter the winner on Tuesday, the day marked a key point as the race turned from one or two contests at a time into a sprawling near-nationwide competition for 419 delegates.
Romney led in the delegate hunt as polls opened. He had 203 in The Associated Press count, while Santorum had 92, Gingrich 33 and Paul 25. It takes 1,144 to win the nomination at the Republican National Convention this August in Tampa, Fla.
The former Massachusetts governor campaigned into Super Tuesday on a winning streak. He captured the Washington state caucuses last Saturday, days after winning a little-contested primary in Arizona and a hard-fought one in Michigan. He won the Maine caucuses earlier in February.
The victories helped settle his campaign, which was staggered when Santorum won a pair of caucuses and a non-binding Missouri primary on Feb. 7.
Santorum and Gingrich have vied for months to emerge as the sole conservative alternative to Romney, and they battered him as a moderate who would lead the party to defeat in November.
But Romney, backed by a heavily financed super PAC, countered Gingrich's victory in the South Carolina primary with a comeback win in Florida. Last week, it was Santorum's turn to fall, as Romney eked out a win in Michigan after trailing by double digits in some polls 10 days before the primary.
Santorum's recent rise has translated into campaign receipts of $9 million in February, his aides announced last week.
Even so, Romney and Restore our Future, the super PAC supporting him, outspent the other candidates and their supporters on television in the key Super Tuesday states.
In Ohio, Romney's campaign purchased about $1.5 million for television advertisements, and Restore Our Future spent $2.3 million. Santorum and Red, White and Blue, a super PAC that supports him, countered with about $1 million combined, according to information on file with the Federal Election Commission, a disadvantage of nearly four to one.
In Tennessee, where Romney did not purchase television time, Restore Our Future spent more than $1 million to help him. Santorum paid for a little over $225,000, and Winning our Future, a super PAC that backs Gingrich, nearly $470,000.
In Georgia, where Gingrich acknowledged he must win, the pro-Romney super PAC spent about $1.5 million in hopes of holding the former House speaker below 50 percent of the vote, the threshold needed to maximize his delegate take.
While the day boasted more primaries and caucuses than any other in 2012, it was a shadow of Super Tuesday in 2008, when there were 20 Republican contests.
There was another big difference, a trend away from winner-take-all contests to a system of allocating delegates in rough proportion to a candidate's share of the popular vote.
Sen. John McCain won eight states on Super Tuesday in 2008 and lost 12 to Romney and Mike Huckabee combined. But six of McCain's victories were winner-take-all primaries, allowing him to build an insurmountable delegate lead that all but sealed his nomination