The race between John McCain and Mitt Romney was approaching the boiling point on Super Tuesday as the candidates hustled to lock up support in the 21 states holding Republican primaries and caucuses.

Though McCain came into the coast-to-coast battle with a healthy lead in the polls, the feud between him and Romney was playing out like a dead heat.

On the Democratic side, Hillary Clinton and Barack Obama were more cautious, recognizing that the 22 states holding Democratic contests Tuesday would likely not settle their one-on-one face-off.

Democratic contests all award delegates proportionally, while nearly half of the Republican contests are winner-take-all. An enormous cache of delegates is at stake Super Tuesday — not enough to clinch a nomination but plenty enough to mint a runaway favorite, or even two. With that appearing more likely on the Republican side, the urgency of those primaries and caucuses could be heard from the leading candidates Tuesday.

Tempers heated up between McCain and Romney, with McCain attacking his opponent for having a “terrible record as governor” of Massachusetts and Romney retorting that he must be in strong contention if he’s so able to get under the Arizona senator’s skin.

The two also clashed over comments Romney made about former Republican presidential candidate Bob Dole, who wrote a letter to conservative talk-show host Rush Limbaugh telling him to ease off his criticism of McCain.

Romney told FOX News Dole is “probably the last person I would have wanted to have write a letter for me.”

McCain demanded an apology, and Romney later tried to call Dole. But Romney said he had nothing to apologize for.

Meanwhile, Focus on the Family founder James Dobson blasted McCain on the Laura Ingraham radio show Tuesday morning, saying, “I cannot, and I will not vote for Sen. John McCain, as a matter of conscience … Should John McCain capture the nomination, as many assume, I believe this general election will offer the worst choices for president in my lifetime. If these are the nominees in November, I simply will not cast a ballot for president for the first time in my life.”

As McCain has picked up steam — winning the New Hampshire, South Carolina and Florida primaries — conservative figureheads have blasted him for being too moderate.

McCain has tried to combat that with a string of endorsements from all points on the partisan spectrum, and by fighting fire with fire.

He rallied in Manhattan on Tuesday morning and took shots at Romney on the talk shows, accusing him of having a “terrible record as governor” and pouring millions of dollars into ads attacking him.

Romney said McCain is “so making up facts that it’s really quite extraordinary.”

He told supporters at West Virginia’s Republican nominating convention Tuesday that McCain’s support for global warming curbs “would effectively kill coal,” a lifeblood of the state. The first round of voting in the state showed Romney in the lead but failed to select a winner.

“This is not a long shot,” Romney said of his candidacy. “I am the candidate who can stop John McCain.”

Weather threatened to be a factor in some states. A wintry mess including snow and ice was forecast for New England, and snow was forecast for a large corridor from southwest Kansas to northern Michigan.

The tightness of the Democratic race and the sheer scale of the voting in nearly two dozen states left Clinton and Obama wary of making predictions as they offered last-minute pitches in a round of early morning network TV interviews.

“We’re all kind of guessing about what it’s all going to mean because it’s never happened before,” Clinton said. “There’s a lot we’re going to find out about how all this works.” She said she found it all “intriguing and somewhat mystifying.”

Obama said a “split decision” was likely. “I don’t think today’s going to end up being decisive,” he said on FOX News. “But I think that our message is starting to break through. And we’re very optimistic about our prospects.”

His campaign manager David Plouffe wrote in a memo Monday that the Obama camp’s strategy is only to stay close enough in the delegate count on Super Tuesday to proceed to the post-Feb. 5 states.

He arrived in Chicago Tuesday afternoon to pick up his wife Michelle and vote.

California has emerged as a key battleground for the Democrats, though, as Obama has recently edged ahead in the polls in the delegate-rich state. Clinton still leads in the polls for the vital New York and New Jersey primaries.

On Super Tuesday, the Iowa-New Hampshire days of retail politicking in rustic diners were a distant memory, although only weeks old. Clinton and Obama each poured more than $1 million a day into TV ads in the last week alone; Clinton bought an hour on the Hallmark Channel for a town hall meeting on Monday night, and Obama saw some $250,000 disappear in 30 seconds in his Super Bowl ad a day earlier.

Not only was the electoral territory vast, but so were the stakes. Romney boomeranged across the country and back in a 37-hour dash, branding himself the true Ronald Reagan conservative at every stop.

Romney campaigned from Tennessee to California on the eve of the voting, then turned back to attend the West Virginia convention. McCain, Obama and Clinton clustered in the hotly competitive Northeast.

Former Arkansas Gov. Mike Huckabee focused on the South, his continued candidacy an open question — as was Romney’s viability if he couldn’t pull off some surprises.

Huckabee and Romney have also been sniping at each other, ever since Romney said a vote for Huckabee is a vote for McCain. National polls show Huckabee running a not-so-distant third behind Romney, and Huckabee’s said he’s still in it to win it.

Back on his home turf of Arkansas, Mike Huckabee counter-punched Romney for telling him to stop “whining” after Huckabee accused him of voter suppression.

“If he thinks this is tough he ain’t seen nothing until he faces Hillary or Obama in the general election, so that’s why I don’t think he’s … the best guy to put in the starting line up for the big game,” he said.

Clinton voted near her home in Chappaqua, N.Y., accompanied by husband Bill and daughter Chelsea. “You’re a Democrat, right?” election worker Evan Norris joked. Clinton smiled. “I am just very excited about today,” she said. “The stakes are huge.”

In Topsfield, Mass., where a steady stream of voters filed to a polling place in a cold rain, teacher Marcia Spector, 58, said she had made the “very, very tough” decision to support Obama, reasoning he would be more able than Clinton to win the presidency in the fall. “I just feel that he is dynamic and he is for change,” she said. “He doesn’t bring the baggage. I think he’s more electable, actually.”

It was tough, too, for Mary Jordan, 43, a teacher’s aide — so tough she said she didn’t make up her mind until she was in the polling booth. Voting Republican, she went for Romney, the state’s former governor, because of his business experience, while offering no one a glowing endorsement. “I think he’s the least unlikable,” she said. “I really didn’t like any of them.”

In Illinois, Obama supporters expressed pride for the home-state senator as they voted. “We have something great to vote for today,” said Catherine Braendel, 44, a marketing consultant who lives down the street from Obama in Chicago.

In Grayslake, Ill., registered Democrat Steve Greenberg, 39, decided his vote would be more valuable on the Republican side as he thought ahead to the general election. “I went with McCain because if the Democrats lost, I’d be more comfortable with him,” he said.

McCain struggled to close the sale with his party’s base after coming strikingly far without its solid support. He said he would extend his hand to Democrats, but “I will preserve my proud conservative Republican credentials.”

Romney sought until the end to exploit the right’s mistrust of McCain, who opposed President Bush’s tax cuts when they were introduced, departed from orthodoxy on immigration, favors mandates to slow global warming and led campaign finance reforms that activists say trampled on their freedom of speech.

McCain responded with a TV ad reminding voters that Romney had changed some stripes. It showed Romney in a 1994 debate calling himself “an independent during the time of Reagan-Bush. I’m not trying to return to Reagan-Bush.”

After months when it was all about expectations and momentum, not to mention confusion, real numbers finally became important.

The two dozen Super Tuesday contests were delivering 1,023 Republican and 1,681 Democratic delegates. The numbers needed to win the nomination: 1,191 Republican and 2,025 Democratic.

John Edwards’ departure after South Carolina’s primary simplified the math but little else on the Democratic side.

Since winning that state, Obama has collected a succession of marquee endorsements — several of them named Kennedy — and pulled into a statistical tie with Clinton in a national poll and in California, Tuesday’s biggest prize with 370 Democratic delegates.

The two were campaigning for history, as well — Clinton seeking to become the first female president, Obama the first black one.

Little separates them on most issues, including universal health coverage, ending U.S. military involvement in Iraq and raising taxes on the rich. And neither has accounted fully for all their proposed spending.

Instead, the campaign has turned on Clinton’s experience and Obama’s vision of change, debated intensely but with more civility in the latest round than when former President Clinton brought racial sensitivities to the surface in stumping for his wife in South Carolina.

The Associated Press contributed to this report.