PHILADELPHIA -- As the Pennsylvania Senate candidates prepare for two crucial debates, the first one here tonight, the question is: why has Democratic Rep. Joe Sestak pulled even with Republican/Tea Party Rep. Pat Toomey?

The answer requires two things: getting the voter-registration numbers and talking to Rep. Bob Brady, the burly guy who runs the Democrats' antique yet still-potent Philadelphia machine.

I did both and the bottom line is this.There may be a GOP wave building elsewhere in the country, but it doesn't show up in the registration figures in deep-blue Pennsylvania; and Brady, a skeptical man who at first was no fan of Sestak's, now thinks "the admiral" can turn it around.

Of course, all elections are about numbers, but in Pennsylvania the arithmetic is stark and simple. The Democrats traditionally have had a huge edge in party registration -- and need to keep it -- and a Democratic statewide candidate needs to "come out of Philly" with at least 400,000 votes.

Chet Harhut, the commissioner of elections, has yet to certify the official final figures, but he and his staff gave me the up-to-the-minute totals and here is the good news for the Democrats: the number of registered Republicans in Pennsylvania is shrinking, not growing. In 2006 there were 3.3 million Republicans in the state; as of today there are 3.1 million. Over that same period, the number of registered Democrats grew from 3.9 million to 4.3 million today.

Both parties have lost adherents since 2008 -- and there has been a small surge in voters who don't register in either party which, Harhut suggested, might reflect the influence of the Tea Party.

But the point is, Pennsylvania remains a Democratic stronghold, and there is no evidence of a GOP surge in registration.

Which means that Sestak's main goal has to be to make the sale to traditional Democrats. It hasn't been easy. He wasn't the establishment Democrrats' candidate in the primary -- Sen. Arlen Specter was. Sestak ran as an outsider, which was an advantage in some places but, paradoxically, not necessarily here, even in a "throw-the-bums out" year.

Pennsylvania is a state with old-school Democrats who grumble about crooked politicians, but distrust corporate power and cling fiercely to the belief that government exists mainly to help working people.

As Brady explained it to me, Sestak now has a chance because he's finally gotten the voters' attention about Toomey -- a true, sincere economic libertarian who wants to dismantle much of the New Deal-Great Society and radically change the tax structure. Pennsylvanians aren't into intellectual experiments.

Brady is a cautious man who doesn't particularly like the spotlight that falls on him at this point in every election cycle. "If we get the 400,000 votes out of Philly, then the candidates take the credit. If we don't, we get the blame."

So he listed reasons why Sestak remains a heavy lift: there is no other Philly-based candidate on the statewide ticket; voters are in a foul, unpredictable mood; President Obama is not popular; when he and other luminaries come through, they generate excitement but "it just doesn't stick."

The only issue is the economy, Brady said, "just like our friend Carville used to tell us." And even though Pennsylvania voters are angry at Washington -- run by the Democrats -- they rely on government and tend to be even more distrustful of corporate power in New York and now overseas.

Two issues may tip the balance. One is Social Security. For the second straight year, seniors are getting no cost-of-living increase, raising the visibility of the program -- always an advantage to Democrats (some of whom are now suggesting special compensatory payments to seniors). Toomey has favored rethinking the program, including making it voluntary for younger workers.

The other issue, Brady says, is the secret-money TV ad campaign run against Sestak by conservative and pro-business groups. Most analysts, including me, considered this too much of a "process" topic for voters to care about.

But, not surprisingly, the pundits might be wrong. Brady, a thoughtful guy who teaches organizational management at the University of Pennsylvania, says that voters are picking up on concerns about where that TV-ad money is coming from. "What if it's coming from some big corporation you deal with every day, and that now is going against you and your interests? What about that?"

Brady, in fact, was the chief author in the House of the Disclose Act, which would have required disclosure of all independent-spending contributions. The measure passed the House but was killed in the Senate.

Most Democrats are avoiding dwelling on the topic, but Brady thinks it's a winner -- and, not coincidentally, the Democratic National Committee is now putting some money into the message.

"We dug ourselves a hole here," Brady said. "But maybe we're climbing out of it."