Some excellent points by AllahPundit:
Would he be a unifying, consensus figure? He voted for TARP, the tax on AIG bonuses, and the auto bailout. Some would forgive him for that given his leadership on the 2012 budget, but some — like the libertarian wing — wouldn’t. Meanwhile, Democrats are planning to use his budget proposal to drive a wedge within the party by forcing a vote in the Senate and making centrist Republicans choke on the Medicare and tax provisions. Collins has already said she opposes his program; doubtless there are others. Imagine a presidential campaign where the candidate’s signature piece of legislation is hit with attack ads showcasing opposition from the moderates in his own party.