In Pennsylvania, Democrats enjoy a substantial registration advantage over Republicans, going well into double digits.  For Democratic presidential contenders, this makes the Keystone State a must-win in a general election; there are few paths to the White House for a Democrat that don’t pass through Pennsylvania.  In 2010, Republicans took control of the state government and won the US Senate seat when Democratic turnout failed to materialize, only edging Republicans by three points in exit polling.

A new poll from Muhlenberg College suggests that Democrats might perform even worse in 2012 — and that Barack Obama may lose Pennsylvania big (h/t Curioser_Georg on Twitter):

President Barack Obama, who political experts say will need a win in Pennsylvania to retain the White House, dipped to 35 percent approval among the state’s registered voters, according to a Muhlenberg College poll released Friday. …

In the Muhlenberg surveys alone, Obama has dropped more than 10 percentage points in less than six months. In March, he held steady with 48 percent. In April 2009, a few months after he started and just after the stimulus bill was passed, Obama was at 61 percent approval in the state.

In the beginning of this month, a Quinnipiac poll had Obama at a 43/54 approval rating and a virtual tie with … Rick Santorum.  His re-elect number was slightly lower than that, at 42%, and the best Obama could do in head-to-head matchups was a 47% against Michele Bachmann in the state.  It’s even worse in the Muhlenberg College poll:

If there is any silver lining in the poll for Obama, it’s that 31 percent of Pennsylvanians say their vote in November 2012 will depend on who the Republican candidate is. And Obama still slightly edges out an anonymous GOP contender 36 percent to 31 percent.

That’s not a silver lining, it’s a pair of lead electoral shoes.  In politics, we have a term for incumbents who can only get 36% in re-elect polls: former officeholders.  Voters already know the incumbents, which is why undecideds break heavily towards challengers.  In a deep-blue state like Pennsylvania, a 36% is going to mean a blowout for Obama in all the wrong ways.

Granted, this is a smallish sample of 380 registered voters (not the more conservative likely voters), but for a statewide sample, it’s acceptable if not robust.  I don’t think anyone from Obama’s team was complaining in March when the same pollster found a 48/44 approval rating.  This is a big warning sign for Obama’s re-election chances, especially if the economy remains stuck in stagnation.  And if Obama’s doing this badly, Republicans have an opportunity to win back the Senate seat they lost to Bob Casey, Jr in 2006.

Update: In late 2010, Democrats had a 50.9/36.9 registration advantage over Republicans, which is why I called it a “deep-blue state.”  It’s one of the largest affiliation advantages Democrats have in the country — but it’s primarily comprised of the white, working-class voters recoiling this year from Obama’s economic policies and performance.  These were the Hillary Clinton voters in 2008 that Obama called “bitter clingers” and lost in the primary.