For Barack Obama to win the Pennsylvania primary April 22, or even to keep the race close, he needs to pull off an extraordinary feat.

That would be identifying sympathetic independent and Republican voters, and persuading them to register as Democrats to vote for him.

The registration deadline is today.

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 in this turbulent race. The state has a large number of the older and blue-collar voters who tend to back Clinton.

Time running out, and trailing by double figures in the polls, the Barack Obama camp has engaged in a last-ditch, house-by-house appeal.

There have been struggles, but elsewhere, signs of progress. Statewide, the number of Pennsylvanians switching affiliation to the Democratic Party has boomed, with 57,651 signing up already this year through March 14.

There is no way of knowing whether these voters are jumping to the party to vote for Barack Obama, but his campaign hopes that is the case.

If turnout for the Democratic Pennsylvania primary is 50 percent, as some analysts expect, these new converts would account for about 3 percent of the total vote - and presumably a significantly larger share of Obama’s tally.

The trend has accelerated. More than 22,000 registered as Democrats during the week of March 10, compared with 7,223 in all of January.

Continue reading in the Los Angeles Times …