If left-wing activists go after Senate Democrats running in 2012 who vote for the tax compromise, it will weaken these incumbents if they hold on to win the primary and make a GOP pickup of some of these targeted seats more likely. If left-wing challengers win some primaries, it is not clear that they could win general election fights in some of the states that moved right in 2010.

This is not to say that Tea Party challengers to GOP Senate incumbents are now over and done with. Richard Lugar of Indiana and Olympia Snowe of Maine are both very popular in their states and would likely be easy winners if nominated for re-election, but they may face challengers from the right. In Indiana, it might not matter who the GOP nominee is — he or she would be favored to win. That is not the case in Maine.

In the new age of demands for greater ideological purity in both parties, the chances for substantive compromises diminish. So the GOP will continue to push taxes lower. And the Democrats will continue to spend. And the deficit will continue to grow. The deficit commission laid out some of what I think could be called substantive compromises on spending and taxes. But senators facing potential challenges from the left or right may be very wary of these kind of compromises, which anger their base.