ELKINS PARK, Pa. — They have known Arlen Specter's name for decades, but they are suspicious of his motivations for switching parties. They know little about Joe Sestak, Specter's challenger in the narrowing race to nominate a Democrat to run for U.S. Senate.

Undecided voters make up a big enough slice of the registered Democrats that many pollsters believe they could tilt Pennsylvania's closely contested primary race in Sestak's favor — if enough of them actually vote.

With two weeks left until the May 18 election, Sestak and Specter are spending millions on TV ads to sway those voters.

Specter for months had led in fundraising and polls. An institution in the Senate and Pennsylvania politics, he was first elected to the Senate in 1980 and his bid for a sixth term is endorsed by President Barack Obama and other party leaders.

Recent polls, however, show Sestak catching up, a sign that voters are increasingly making up or changing their minds as they find out more about Sestak through recently aired television ads.

Undecided voters who show up at the polls are more likely to support Sestak, pollsters say.

"If they haven't made up their mind yet about Arlen — and many have known him for ages — are they now going to vote for him after all these years?" asked G. Terry Madonna, a pollster at Franklin & Marshall College in Lancaster.

A Quinnipiac University poll that surveyed likely Democratic voters through Sunday showed Sestak trailing 47 percent to 39 percent, a month after he trailed by 21 points. Fourteen percent of voters remain undecided, and more than one-third of those who picked a candidate also say their decision is not final.

Specter, a longtime moderate, switched parties a year ago, saying he had irreconcilable differences with the party over his vote for Obama's economic stimulus plan. He also acknowledged that chances were slim that conservative Republicans who tend to vote in primaries would nominate him again for his seat.

Sestak is hammering Specter votes that went against Democratic causes. For instance, in 2007, Specter and 45 other GOP senators tried unsuccessfully to block Democrats from insisting on a troop withdrawal from Iraq as part of a wider bill.