Is the United States Heading for a Constitutional Crisis?

In a post entitled Senator Shelby and the Emerging Constitutional Crisis of our Time, Jack Balkin writes that Sen. Shelby's blanket holds,

together with Scott Brown's election in Massachusetts, should be a wake up call to President Obama that he faces a political and legal crisis of the first order. The Senate's rules, which are not required by the Constitution, need to be reformed immediately or else day-to-day governance threatens to become impossible.

This crisis is not yet technically a constitutional crisis, because the Senate's rules are not constitutionally required. But if the President does nothing, and argues that there is nothing he can do to persuade the Senate to change its mind because the Senate gets to determine its own rules under Article I, section 5, we face what Sandy Levinson and I have called a Type Two constitutional crisis -- in which acceptance of the political rules of the game sends the country over a cliff.

Update: In the New York Times, Paul Krugman makes a similar argument that the abuse of Senate rules for obstructionist purposes is "making the nation ungovernable."