Washington - As a passionate, divisive, 20-month US election enters its final two weeks, Cara Tobias-Ingram remains one of the undecided voters who could determine whether Barack Obama or John McCain make it into the White House.

Economically conservative but socially liberal, Tobias-Ingram finds every US election hard to gauge. Her husband sits firmly in Republican McCain's camp. Her two sisters vouch for Democratic nominee Obama.

Tobias-Ingram, a native of Portland, Oregon, herself voted for Democrat Al Gore in 2000 but President George W Bush in 2004.

"I know, it's just mindboggling to understand," said Tobias-Ingram of her indecision, during a McCain campaign rally in Woodbridge, Virginia. "I empathize with both positions."

She is what everybody in the late frenzy of the campaign calls a swing voter, and she's not alone.

Opinion polls have given Obama a clear edge in recent weeks over McCain, attributed largely to the debate shifting to the faltering US economy and domestic issues that tend to favour Democrats, especially with a Republican president wallowing in disapproval after nearly eight years in office.

But the remaining undecided voters are part of the reason Obama still has trouble topping 50 per cent. An average of all national polls compiled by realclearpolitics. com on Tuesday gave Obama 49.3 per cent to McCain's 44.1 per cent.

Independent-minded voters like Tobias-Ingram are exactly where both campaigns will be focussing their efforts in the last two weeks.

To narrow it further, all attention will be on the unconvinced in battleground states, which hold the keys to victory given the US' unique, winner-take-all electoral system. Win a state by just one vote, and a candidate still gets all of its electoral college votes, making the most closely divided states the biggest prizes in any election.

The trend currently favours Obama in many of the usual suspects. Polls put Obama ahead - though not by much - in most of the major swing states that helped Bush eke out victory in 2000 and 
2004, including Florida, Ohio and Missouri.

Meanwhile, Obama has maintained a strong edge in swing states that voted for Democrat John Kerry in 2004, including Pennsylvania and Michigan.

The electoral map has shifted significantly since 2004. The centre-left Democrats have made inroads in a number of recently Republican regions, including Southern states like Virginia and North Carolina and Western strongholds including Colorado, Nevada and New Mexico.

The result: In the decisive electoral college, where states are weighted by population, Obama already has enough states leaning in his favour to capture the White House if the results bear out on election day, according to realclearpolitics. com.

"Since (McCain) is clearly behind nationally and in virtually all of the battleground states, he's still looking for a game changer," said Thomas Mann, an elections scholar and senior fellow with the Brookings Institution, a centre-left think tank.

The shift toward the Democrats is fueled by a combination of factors. Some of it is because of national issues. Polls across the country favour Obama on the economy, which has leapfrogged all other factors in the public consciousness since last month's meltdown of financial markets.

The already sluggish economy, impatience with the Iraq war and disaffection for Bush have all made it a tough climate for McCain.

Since the intra-party nomination process, which started in January, new voter registrations have disproportionately boosted Democrats, while Obama's record-shattering campaign fundraising has allowed him to blanket swing states with television advertising.

There are more localized reasons for the shift, too, the result of changing demographics.

Western mountain states, for example, have witnessed an influx of both minority Hispanic voters and left-leaning white college graduates, two key voting groups that tend to favour Democrats. The proportion of working-class voters who trend Republican has declined, according to a Brookings Institution analysis.

Virginia, which hasn't by won by the Democratic presidential nominee since 1964, has found less conservative newcomers moving into its northern region just outside the US capital Washington. Obama now leads the state by an average of 8 percentage points.

Meanwhile, working-class voters in the so-called Rust Belt, an area hit hard by declining heavy industry, including Ohio, Michigan and Indiana, have borne the brunt of a slowing economy, drawing them to Obama's simple message of change from the Bush administration.

With two weeks left, McCain, Obama, their running mates and allies will be campaigning furiously across swing states. Early voting already began this week in several important states including Colorado, Nevada, New Mexico and Florida. More than 30 per cent of voters may have already cast their ballots by November 4. (dpa)