JAMES TARANTO: The Senate Gets MAD: The politics and psychology of the “nuclear option.”

Majority rule is always in the immediate interest of the majority party. But there are three countervailing incentives that have stopped majority senators from supporting a change in the rules: ideological moderation, concern for the institutional power of the Senate, and long-term self-interest. The first two of those incentives have gradually weakened for political reasons. The third suddenly broke down for perverse psychological reasons.

Each party used to have a fair number of ideological outliers: liberal Republicans and conservative Democrats, or “moderates” if you prefer. Most of the Gang of 14 fit this description: They included liberal Republicans Lincoln Chafee and the Maine Ladies as well as Democrats Ben Nelson, Joe Lieberman and Mark Pryor. Of those six, only Pryor and Susan Collins are still in the Senate. Of the 14, only five are.

Of the nine who’ve departed, eight have been replaced by either liberal Democrats or conservative Republicans. The lone exception, Robert Byrd of West Virginia, was replaced by Joe Manchin, one of the three Democrats who voted against the nuclear option. Pryor was also among the dissenters. The general trend, in the Senate as in the country, has been toward ideological polarization of the parties.

The end of the filibuster entails a serious diminution of the Senate’s power vis-à-vis the president and the House. As we observed this July, the Senate’s power consists largely in its ability to withhold consent from both House-passed legislation and presidential actions (nominations and treaties). Thus majority rule enhances the power of the majority party at the expense of every individual senator, regardless of party.

As the Senate has become more partisan, and members elected during an earlier age have retired or died, concern for the Senate’s institutional power has diminished. Yesterday’s third Democratic dissenter, Carl Levin of Michigan, is one of only three remaining Democratic senators first elected before 1984.

There’s less institutional memory than there used to be.