Rep. Joe Sestak, D-Pa., right, talks to voters at a campaign stop in Elkins Park on April 25. Democratic voters who are still undecided over whom to support in the May 18 primary between Senate candidates Arlen Specter, a former Republican, and Sestak might decide the outcome of the closely contested race.

WASHINGTON — Month after month, Rep. Joe Sestak spoke at campaign events with only a handful of attendees. Poll after poll showed him badly trailing Sen. Arlen Specter.

Whenever he was asked about the grim reality of his struggling campaign, Sestak would smile and say a version of: "It will come down to the last week."

Looks as though he was right.

A week before Tuesday's Pennsylvania Democratic primary for U.S. Senate, Sestak finds himself ahead in more than one poll. He caught Specter on May 7, a Morning Call/Muhlenberg College tracking poll shows, and since then has built a small lead.

The latest numbers, made public Monday night, were Sestak 47, Specter 43.

"I think that this race, which so many people were writing off a month ago, is playing out exactly as Joe Sestak and his team planned," said Mark Nevins, a Democratic consultant in Philadelphia. "Sestak has (Specter) right where he wants him."

Sestak's late surge has coincided with his television ads. When the race started, few voters outside his Delaware County congressional district knew of the former admiral. Pennsylvanians, by contrast, have known of Specter for years.

Despite having plenty of money, Sestak waited until the closing weeks of the campaign to air ads. And he waited until May 5 to pull out a showstopper — footage of former Republican Specter alongside George W. Bush, Rick Santorum and Sarah Palin.

If the polls are the indicator, the ad seems to have resonated with Democratic voters. Specter admitted Monday at an event in Allentown that the race is tightening and attributed it to Sestak's vigorous advertising — "$1.6 million in the last week."

Speaking before about 100 people at the International Brotherhood of Electrical Workers union hall, Specter said tighter polls mean fresh enthusiasm. "I have seen people who are very concerned about keeping me in the Senate here," Specter told the crowd. "And it is a real battle."

Roger Lund, a Sestak supporter and the chairman of the Adams County Democratic Committee, reads the ad war another way: Specter hurt himself with his own ad declaring Sestak had "created a poor command climate" in the Navy.Some undecided voters, Lund suspects, took offense at the negativity of that ad and are backing Sestak "even if they're not madly in love with (him)."If TV ads tightened the race, perceptions of Specter — and what to make of his party switch — are likely to decide it. Election Day, experts say, will be a referendum on Specter.Specter said Monday he's planning his own late television push that will include an ad featuring President Barack Obama, who remains very popular among Democrats."We have a pretty good arsenal," Specter said, "and we are going to be spending a fair amount of money this week on television ourselves."People who vote in primaries tend to be party loyalists. The lower the turnout, the better the chances for Sestak, said Chris Borick, pollster at Muhlenberg College in Allentown. Lifelong Democrats might be unable to push the button for Specter, a Republican officeholder for more than 40 years and a senator since 1981.Still, as the Democrat endorsed by party leaders, Specter will benefit on Election Day from get-out-the-vote resources employed by both the state and national parties.But there also is a grass-roots effort afoot for Sestak. His supporters liken it to tactics that helped Obama win over party pick Hillary Clinton in the 2008 Democratic presidential primary.Monica Kline, an alpaca farmer in Lebanon County, was a delegate for Obama two years ago and said she feels the same groundswell this week for Sestak.

Sen. Arlen Specter, D-Pa., shakes hands with Vice President Joe Biden, right, after a campaign rally for Specter on April 23. Specter, who held office for more than 40 years as a Republican, might have a hard time convincing some Democrats to vote for him.