But a McCain victory is also within the realm of possibility. ABC News' analysis calculates that McCain needs to secure wins in five toss-up states, all of which went to President Bush in 2004, to win. In addition to taking Ohio, Florida, Missouri, Indiana and North Carolina, McCain will also need to pick up some states now leaning in Obama's direction, such as Pennsylvania and New Hampshire.
Still, Monday afternoon, McCain pollster Bill McInturff warned that exit polls could exaggerate Obama's lead.
"We would discourage a rush to judgment based on the exit polls and wait until there has been a representative sampling of actual tabulated results from a variety of counties and precincts in a state," he said in a statement.
He added, "It is not just the national exit poll that skews Democratic, but each of the state exit polls also suffers from the same Democratic leanings."
On "Good Morning America" this morning, NPR's senior political analyst Juan Williams said suburban white women in the Midwest will be critical to McCain's effort.
"John McCain needs those women in Pennsylvania, Ohio and Missouri to come out in big numbers and support him and they've gotta break his way if he has a chance today," Williams said.
According to ABC News' analysis, 18 battleground states with a total of 194 electoral votes are the states to watch this Election Day.
Four of those 18 battlegrounds were leaning Republican heading into Tuesday: Georgia, West Virginia, North Dakota and Montana.
That leaves another five, worth a total of 84 electoral votes, deemed true toss-ups, according to ABC News' assessment.
INDIANA: 11 Electoral Votes
Of those critical states, Indiana's polls close first. Most of the polls in Indiana close at 6 p.m. Eastern time, and the last of them close at 7 p.m. Eastern.
Indiana has not gone to a Democrat in a presidential election since 1964. But polls released Thursday show a dead heat between Obama and McCain in the Hoosier state at 47-47, according to a Research 2000/WISH-TV poll. Another Thursday poll from Selzer & Co./Indianapolis Star/WTHR has Obama leading by one point among likely voters, 46-45.
Signaling Indiana's importance to the race, Obama plans to visit the state today in his 49th campaign trip there.
Strategists say the key area to watch in Indiana is Indianapolis and its suburbs. For Obama, pockets of opportunity may be found in the northwest corner of Indiana, close to Chicago, where towns like Gary and South Bend tend to vote Democratic. He also hopes to find support in Perry County in the south, as well as in Bloomington and West Lafayette, home to Indiana and Purdue universities.
"Some of the states that close early -- Indiana, Virginia, North Carolina, Florida -- we've certainly targeted," said Obama's campaign manager David Plouffe on Tuesday on "Good Morning America." "But we think we've got a lot of different ways to get there."
Meantime, Monday, McCain told an Indianapolis crowd, "Indiana is now a battleground state, but it's a battle that we're going to win."
NORTH CAROLINA: 15 Electoral Votes
North Carolina voters have until 7:30 p.m. EST to cast their votes.
According to an NBC/Mason-Dixon poll released Sunday, McCain leads 49-46 in the Tarheel State. North Carolina has not voted for a Democrat for president since 1976.