The combined electoral votes in Ohio, New Mexico and Iowa would be enough to put Obama over the 270 Electoral College votes if there are no major upsets by McCain.
In Chicago's Grant Park, where Obama was expected to speak, about 65,000 supporters cheered the projections as they were announced. Up to one million more people were expected to throng the streets around the lakeside park to celebrate if Obama wins.
John Nichols, Washington correspondent for The Nation newspaper, told Al Jazeera that the Ohio projection would be "heartbreak" for the Republicans.
"Many Democrats think it was the loss of Ohio that cost them the 2004 election," he said.
"Why does it mean something? It's in the middle of the country, it's rural, urban ... if you win Ohio your chances of winning the presidency are pretty good.
"McCain put his lifeblood into this state, fought county by county, city by city. He really focused on here, he knew he had to, no Republican has ever been elected president without winning Ohio."
After the Ohio projection, Jesse Jackson Jr, the Obama campaign co-chair, said: "Barack Obama as the 44th president ... it is mindboggling, mindblowing.
"Our children walk into a very different America tomorrow," he told Al Jazeera.
"For some people it is the end of a two-year campaign and for others it has been a 200-year trek ... The civil rights movement, among others, have made this moment possible."
Earlier, media projections had indicated that Obama would take the 21 electoral votes in Pennsylvania, which had been targeted by the Republicans.
"The road to victory for John McCain was to save all of the states President Bush won in 2004 and then try to snatch away a state like Pennsylvania or New Hampshire in order to offset loses in the west later on," Al Jazeera's Rob Reynolds, reporting from Chicago, said.
NBC and Fox News both also projected an Obama win in New Hampshire.
Obama has retained a lead in most nationwide opinion polls and had the advantage in most of the battleground states, which were expected to decide the election.
But John McCain, his Republican rival, had dismissed the opinion polls and promised an upset.
"First of all I want to say I really think we should wait for the election to be determined officially," Douglas Holtz-Eakin, the McCain campaign economic adviser, told Al Jazeera as the projections came in.
"We believe that John McCain has outperformed the exit polls consistently and still has a good chance."
A record number of citizens were expected to cast votes in Tuesday's poll.
Laura Egerdal, communications director for Missouri's secretary of state, said turnout was "unprecedented".
"I think today we are easily going to set a record for the sheer number of voters turning out today."
Al Jazeera's Sarah Brown, reporting from Chicago, said: "There are some incredible scenes with lines of voters queueing around the block at some polling stations.
"At some places people started lining up at 5am and had to wait up to two hours to vote. As expected, Obama's hometown is doing him proud - of more than 30 voters polled informally all but one were for the Democratic candidate."
Jean Jensen, Virginia's secretary of state, said the turnout was "phenomenal", noting that up to 40 per cent of the state's registered voters had cast their ballots by 10am.
Polling was extended in two areas of North Carolina in order to accommodate people still queuing to vote.