WASHINGTON -- Democrat Barack Obama heads into Super Tuesday's voting with apparent momentum in several big states where Hillary Clinton has been the longtime leader, raising hopes in his camp for upsets that could give him the edge in delegates and enthusiasm coming out of the primaries' biggest voting day.
Republican John McCain, meanwhile, maintains solid leads in the Northeast but faces scattered threats from Mitt Romney in the South, Midwest and particularly in California.
Possible late swings by blocs of voters raised the uncertainty level nationwide as nearly 80 million Americans will be eligible to vote today for the Democratic or Republican presidential nominations in 24 states.
For Democrats, the key question was whether Clinton and Obama would split the delegates up for grabs or whether one of them would gain a decided advantage. All Democratic primaries and caucuses will award delegates on a proportional basis.
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Late polling suggested that Illinois Sen. Obama was gaining on New York Sen. Clinton in several big states where she had enjoyed solid leads, among them California and New Jersey.
"Barack Obama enjoyed a big Sunday bounce in important Democratic contests," said pollster John Zogby, who surveyed those states for Reuters and C-SPAN. "He pulled into a tie in New Jersey. He pulled into the lead in Missouri, and he slightly grew his lead in California."
The Polling Institute at Quinnipiac University in Connecticut also found Obama gaining in the East, narrowing the gap in Clinton's home state of New York and pulling within reach in New Jersey.
"Senator Obama has put together a coalition of blacks and independents to dramatically narrow the gap in just the last few days," said Clay Richards, the assistant director of the Quinnipiac poll. "The big question remains: Will there be enough of these new, young first-time voters showing up to pull a surprise?"
For Republicans, the critical question is whether Arizona Sen. McCain will sweep most of the contests today and all but wrap up the nomination -- or whether former Massachusetts Gov. Romney will rally enough disgruntled anti-McCain conservatives to keep his bid alive.
"It's hard to envision McCain not walking way from Tuesday as the big winner," Zogby said. "The suspense is around California. ... If Romney wins California, the conservatives have a choice. "
Here's a guide to some of the key states in play today, moving from East to West to match the times the polls will close:
EAST (Six states, with 23.6 million voters)
• Massachusetts. Clinton leads most polls by an average of 14 points, but one recent survey found Obama up by 2. Romney leads by an average of 22 points in his home state.
• New Jersey. Clinton has an average lead of 7 points in recent polls. But one poll found a tie, and all suggested that Obama has gained there. McCain has a big lead, an average of 24 points.
• New York. Clinton led by 14 points in the most recent Quinnipiac poll. That's a solid lead, but she led by 26 in late January. McCain leads by about 30 points, a margin that has grown since former New York Mayor Rudy Giuliani endorsed him.
Also voting: Connecticut, Delaware, West Virginia.
SOUTH (Five states, with 13.7 million voters)
• Alabama. Clinton and Obama are essentially tied. McCain leads former Arkansas Gov. Mike Huckabee by an average of 7 points in recent polls, with Romney far behind.
• Georgia. Obama has a solid lead, thanks to big support from blacks. McCain, Romney and Huckabee are locked in a close three-way fight.
• Tennessee. Clinton leads by an average of 13 points in the few polls done there. McCain and Huckabee are very close, with Romney not far behind.
Also voting: Arkansas, Oklahoma.
MIDWEST (Five states, with 16.1 million voters)
• Illinois. Obama leads in his home state by an average of 32 points. McCain's up an average of 14 points.
• Missouri. Democrats are neck and neck, with Obama up slightly in a Reuters-CSPAN poll and Clinton up slightly in a McClatchy-MSNBC poll. McCain has an edge among Republicans, but Huckabee and Romney are neck and neck, 7 points back.
Also voting: Kansas, Minnesota, North Dakota
WEST (Eight states, with 25.5 million voters)
• Arizona. McCain is up by 16 in his home state -- the weakest home-state standing among top candidates. Clinton's up by an average of 6 points, though by just 2 in last weekend's McClatchy-MSNBC poll.
• California. Obama and Clinton are neck and neck. He had the edge in three out of the four most recent polls -- a marked change after she had led in dozens of earlier polls. McCain and Romney also are neck and neck.
Also voting: Alaska, Colorado, Idaho, Montana, New Mexico, Utah.