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WASHINGTON (Reuters) - Some of the earliest returns in Tuesday's U.S. presidential election could provide big clues about the outcome.

Trends in the race between Democrat Barack Obama and Republican John McCain could become clear soon after the first polls begin to close at 6 p.m. EST in Indiana.

Obama and McCain are locked in a surprisingly tight duel in Indiana, a Midwestern state that has voted Republican in every White House race since 1964. A breakthrough win for Obama, or even a neck-and-neck struggle, would be an encouraging sign of broad strength for the senator from neighboring Illinois.

But if McCain appears to be cruising to a relatively easy win in Indiana it could signal trouble for Obama, who is challenging McCain 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 2 or 3 points, it probably suggests a big Obama win nationally. If it's more than 4 points for McCain, it's going to be wait and see for a while."

The next round of tests is at 7 p.m. EST 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," Schoen said. "If McCain wins by more than a few points that could suggest movement toward him."

At 7:30 p.m. EST, polls close in the states of Ohio and North Carolina.

By 8 p.m. EST, all polls in Florida will be closed. Florida's 27 electoral votes and Ohio's 20 electoral votes are two of the biggest prizes still up for grabs on Tuesday.

MCCAIN MUST WIN THEM ALL

McCain, an Arizona senator who faces a perilous path to gaining the 270 electoral votes he needs to win, essentially has to carry all of those early battleground states to have a realistic chance.

A setback in any would increase pressure on McCain to make up for the loss with an upset of Obama in Pennsylvania, which Democrats have taken in the past four presidential elections. Voting in Pennsylvania, which has 21 electoral votes, also ends at 8 p.m. EST.

The presidential race is not the only battle with an early bellwether. The first returns could offer hints about the fight for control of the U.S. Senate as well.

Democrats are expected to dramatically boost their narrow 51-49 control on Tuesday but need to pick up nine seats to reach a 60-seat majority that would give them the muscle to defeat Republican procedural hurdles.

The first crucial Senate showdown is in Kentucky, where Senate Republican Leader Mitch McConnell is in a tough fight for re-election and, like Indiana, polls begin to close at 6 p.m. EST.

"Indiana offers an early tip about the presidential race, and Kentucky will do the same for the Senate," said Peter Brown, assistant director of the Quinnipiac University poll.

"If McConnell wins Kentucky, there is no way the Democrats get to 60," Brown said. "We're going to know two things fairly early -- whether it's a presidential blowout and whether the Democrats have any shot at 60 seats. Those are two big questions."