DOYLESTOWN, PA. — After knocking on doors at a half-dozen houses, Mardi Harrison, a campaign volunteer for 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 the 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 there had any use for Obama. "Yeah, you have the wrong house!" she said. And she shut the door.
Obama trails Hillary Rodham 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 this month's most favorable poll for Obama shows her leading by 11 percentage points. One poll has 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 as Democrats. The registration deadline is Monday.
With time running out, the Obama campaign is engaged in a house-by-house appeal.
While Harrison and other volunteers often get a cool reception, there are some signs of progress. Statewide, the number of Pennsylvanians switching affiliation to the Democratic Party has boomed, with 57,651 signing up this year through March 14.
There is no way of knowing whether these voters are jumping to the Democratic Party to vote for Obama, but his campaign hopes that is the case. If turnout for the primary is 50%, as some analysts expect, these new converts would account for about 3% of the total vote -- and presumably a larger share of Obama's tally.
And the trend has accelerated. More than 22,000 registered as Democrats during the week of March 10, compared with 7,223 in the entire month of January.
"I've never seen numbers like this in one week," said Leslie Amoros, a spokeswoman for the Pennsylvania Department of State.
For Clinton, Pennsylvania is a must-win. With 158 delegates, the state is the largest of the 10 that have not yet voted. After nearly three months of primaries and caucuses, Clinton trails Obama by about 120 delegates.
An impressive victory in Pennsylvania would narrow that deficit while buttressing Clinton's claim that Obama is a flawed candidate who cannot close the deal with Democratic voters. It would also help make her case that she is better positioned to win the important swing states in November.
