Trends in the race between Democrat Barack Obama and Republican John McCain could become clear soon after the first polls begin to close at 6pm in Indiana (7am on Wednesday, Singapore time).
The two are locked in a surprisingly tight duel in Indiana, a mid-western state that has voted Republican in every White House race since 1964.
A breakthrough win for Mr Obama, or even a neck-and-neck struggle, would be an encouraging sign for the senator from neighbouring Illinois.
But if Mr McCain appears to be cruising to an easy win in Indiana, it could signal trouble for his rival, who is challenging the war veteran in about a dozen states won in 2004 by Republican President George W. Bush.
The first public sign of Democrat John Kerry's loss in 2004 came from a worse-than-expected 20-point blowout in Indiana.
'If Obama wins Indiana, the election is over,' Democratic consultant Doug Schoen said.
'Even if it's close, within two or three points, it probably suggests a big Obama win nationally. If it's more than four points for McCain, it's going to be wait-and-see for a while.'
The next round of tests is at 7pm when voting ends in Georgia, parts of Florida and the battleground state of Virginia - another place where Democrats have not won a presidential vote since 1964 but have made gains in recent statewide races.
'If Obama wins Virginia by a decisive margin, it's a pretty strong suggestion he's going to win the election,' Mr Schoen said.
Mr McCain has to win all of those early battleground states for a realistic chance of winning the election.
The first returns could also offer hints about the fight for control of the US Senate. Democrats are expected to dramatically boost their narrow 51-49 control but need to reach a 60-seat majority to give them the muscle to defeat Republican procedural hurdles.
REUTERS