Two new polls showed Pennsylvania's Democratic Senate primary tightening as the campaign entered its final two weeks Tuesday, the outcome likely to hinge on TV advertising barrages and the mysteries of voter turnout in a midterm election.

Sen. Arlen Specter led Rep. Joe Sestak 47 percent to 39 percent among likely primary voters in a Quinnipiac University survey, but the incumbent's advantage had shrunk 13 percentage points from the school's last poll April 7.

The poll, which was conducted Friday through Sunday, comes three weeks after Sestak began running statewide television ads, raising his profile beyond his Delaware County-based congressional district.

A Muhlenberg College/Morning Call daily tracking poll Tuesday showed an even closer race, with 46 percent of likely Democratic voters surveyed backing Specter and 42 percent behind Sestak. Eleven percent were undecided.

"Mothers tell their children that money doesn't buy you happiness," said Peter A. Brown, assistant director of Quinnipiac's Polling Institute. "Perhaps, but money buys name identification, and that can buy political victory."

Sestak has spent an estimated $2.4 million on broadcast and cable TV ads since April 21, compared with $2.9 million for Specter.

Specter's campaign also got an infusion of about $407,000 over the last week from the Democratic Senatorial Campaign Committee to augment its own ad spending - one of the advantages of being endorsed by the White House and the entire party establishment. The DSCC has committed about $700,000 to the Specter effort, Democratic sources said.

Specter, 80, is seeking a sixth term, running for the first time as a Democrat. He switched from the Republican Party last year after his crucial vote for President Obama's $787 billion stimulus caused a backlash among the Republican base that threatened his ability to win a primary.

Sestak, 58, is a retired Navy admiral in his second term representing the Seventh District in the U.S. House. He argues that he is the true Democrat in the race, and refused to get out when Obama and Gov. Rendell endorsed Specter.

Still, more Democrats surveyed by Quinnipiac said that Specter shared their values (41 percent) than did Sestak (34 percent). Specter's rating on the question was unchanged from the school's previous poll, while Sestak's jumped eight points. The poll also found that respondents trusted Specter (42 percent) more than Sestak (31 percent) to do what he says he will do, a slight narrowing of the margin found in early April.

Fourteen percent of likely Democratic voters were undecided, Quinnipiac found. Although some certainly won't vote, Brown said, "history tells us that incumbents get relatively few undecided voters at the end."

Despite the narrowing of the horse race, Quinnipiac found that Democrats think Specter is more likely to win in November against Republican Pat Toomey, 60 percent to 23 percent, a potential advantage for the incumbent as voters weigh their choices.

The Quinnipiac survey was based on telephone interviews with 930 Democrats who have voted in recent primaries, with a margin of error of plus or minus 3.2 percentage points.

Muhlenberg interviews about 402 likely Democratic voters each day, with the results subject to a margin of error of plus or minus 5 points.

Specter has consistently hovered just above or below the 50 percent line in most polls, an indication of the difficult political climate for incumbents this year. That gives Sestak an opening, analysts agree.

"It's hard to take out an incumbent, and in a primary it's even harder," said Villanova University political scientist Lara M. Brown, who is not related to the pollster. "If you're going to do it, this is probably the perfect situation: a midterm election with a lot of anti-incumbent sentiment, running against a party-switcher."

Sestak, however, has not yet used his TV time for attack ads reminding Democrats of Specter's Republican past. Both sides expect it's coming.

"We're going to see an intense campaign in the last 10 days, in money and negativity," Lara Brown said.