Barely one year after Pennsylvania Sen. Arlen Specter announced his party switch, one question endures: Can the former longtime Republican win the Democratic primary next Tuesday?

After switching parties in April 2009 to run for re-election as a Democrat because he said he could not win the GOP nomination, Specter has blown a lead of more than 20 points over Rep. Joe Sestak (D) in just a few weeks, and polls show him tied with or trailing the sophomore Congressman.

So what happened?

“Sen. Specter was a known commodity. Sestak was the unknown commodity,” Montgomery County Democratic Committee Chairman Marcel Groen said. “Once he went on television, we figured it would tighten up quickly.”

Polls have shown that likely Democratic voters already know Specter, making Sestak, who was relatively unknown for most of the statewide campaign, the only candidate with room to grow. A Quinnipiac University poll released last week showed Specter with an 8-point lead over Sestak, but 9 percent of likely primary voters had not heard of Specter compared with 44 percent who were unfamiliar with Sestak.

“I expected it to be very close, very tight,” Groen added. “At the end, Sen. Specter will end up prevailing by somewhere around 6 points. Nobody ever expected him to win by 22 points.”

But at least one local Democratic operative said Specter simply did not play his cards right by going on the air too late against Sestak. Pennsylvania Democratic consultant Mark Nevins said Specter “essentially negated” his own cash lead in the primary — more than $3 million at the end of March — by hitting statewide airwaves just one week before Sestak.

“I thought the window for Joe Sestak had almost closed, and it seems like he got a foot in there and kept it wedged open, and now it’s wide open and he climbed right through it,” Nevins said.

By saving his more than $5 million in campaign cash for the last four weeks of the primary, Sestak could maximize his saturation in the state’s many media markets.

“I think that the Specter campaign made a tactical error in allowing the Sestak campaign to condense this primary campaign into a four-week sprint instead of a three-month engagement,” Nevins continued.

Several local strategists and party operatives said Specter’s saving grace may lie with black voters in urban Philadelphia.

Pollster Chris Borick, whose daily Muhlenberg College/Allentown Morning Call tracking poll gave Sestak a 5-point lead Monday, said black voters are one of the toughest constituencies to survey in any race. He said his poll of about 400 likely Democratic primary voters includes an average sub-sample of 60 to 70 black respondents.

But what makes black turnout even more unpredictable next week is the number of black voters who registered for the 2008 presidential primary in Pennsylvania — a constituency that President Barack Obama won in a landslide despite losing the state to then-Sen. Hillary Rodham Clinton (N.Y.) in the Democratic primary.

“If those African-Americans did turn out, which might not be captured in a lot of the likely voter models that we use and a lot of others use, it might absolutely impact the final results,” Borick said.

One Democratic source expected Obama to visit Philadelphia this weekend in a last-minute push for Specter. The White House press office did not respond to an inquiry about the potential visit, but Pennsylvania Gov. Ed Rendell (D) hinted at an upcoming intervention from Obama, Vice President Joseph Biden and Philadelphia Mayor Michael Nutter on a recent MSNBC appearance.

“And remember, you haven’t heard from President Obama,” Rendell said on Friday. “You haven’t heard from Joe Biden. You haven’t heard from me. I just did some robocalls. Mike Nutter has been pretty outspoken in the city. And I think you’re going to see those people weigh in. … Those are the leading Democrats when it comes to voters throughout the state and in particular in the vote-rich southeast.”

Specter’s campaign also cut a radio spot featuring Obama that is running on Philadelphia radio stations, and Philadelphia Phillies shortstop Jimmy Rollins has done robocalls on behalf of Specter.

“This campaign has been so predictable,” Pennsylvania Democratic consultant Larry Ceisler said. “There have been no surprises here. You just know which ad is coming and you gotta believe in the next week, the Obama [ad] comes for Specter. You’ll be able to believe he put a lot of money behind that ad.”

Specter’s fate might be tied to the success of another statewide Democratic candidate: state Sen. Anthony Williams (D), who is running for governor. Sources said that if Williams, who is black, performs well with his Philadelphia base, Specter will get a boost from turnout.

“Specter’s campaign is somewhat predicated on the success of Tony Williams turning out African-American voters in his bid for governor,” Ceisler said.