The only reason he’s doing this is that he knows he can’t win in the Republican primary and has a better chance running against the other Democrats in their primary:

When I supported the stimulus package, I knew that it would not be popular with the Republican Party. But, I saw the stimulus as necessary to lessen the risk of a far more serious recession than we are now experiencing.

Since then, I have traveled the State, talked to Republican leaders and office-holders and my supporters and I have carefully examined public opinion. It has become clear to me that the stimulus vote caused a schism which makes our differences irreconcilable. On this state of the record, I am unwilling to have my twenty-nine year Senate record judged by the Pennsylvania Republican primary electorate. I have not represented the Republican Party. I have represented the people of Pennsylvania.
I have decided to run for re-election in 2010 in the Democratic primary.
[…]
My change in party affiliation does not mean that I will be a party-line voter any more for the Democrats that I have been for the Republicans. Unlike Senator Jeffords’ switch which changed party control, I will not be an automatic 60th vote for cloture. For example, my position on Employees Free Choice (Card Check) will not change.