Doylestown, Pennsylvania:  After knocking on doors at a half-dozen houses, Mardi Harrison, a campaign volunteer for Illinois Senator Barack Obama, finally found someone to listen to her pitch.

Anyone who wants to vote for Obama in Pennsylvania's primary must be registered as a Democrat, she explained to a woman who answered the doorbell. Did the independent voter at this address want to sign up?

The woman laughed and made it obvious that no one inside had any use for Obama. "Yeah, you have the wrong house!" she said. And she shut the door.

Obama is trailing New York Senator Hillary Clinton by a large margin in Pennsylvania, site of the next Democratic presidential contest.

The state has a large number of the older and blue-collar voters who tend to back Clinton. Even the most favourable poll in March shows her leading by 11 percentage points. One poll had her ahead by 26.

For Obama to win the April 22 election, or even to keep the race close, he needs to pull off an extraordinary feat: identifying sympathetic independent and Republican voters and persuading them to register this time as Democrats. The deadline to switch parties is today.

Spending

Financial edge

Barack Obama has built a huge financial edge over Hillary Clinton as the White House hopefuls gear up for a dogfight ahead of next month's crucial primary clash in the state of Pennsylvania.

The latest monthly financial reports to the Federal Election Commission show Obama raking in $55 million (Dh202 million) in contributions in February, with less than $1 million in debt.

New York Senator Clinton raised $35 million in February, but has outside debts reported at $3.7 million - not including the $5 million of her own money she lent her campaign.

Obama had nearly $39 million on hand at the end of February, compared with $33 million for Clinton, and the Obama campaign outspent his rival $41 million to $30 million in February, even though he burned through funds at a lower percentage of contributions than his rival.