It follows that each of those two parties must be a widely drawn coalition of people who can agree on a bare minimum of policies, but whose main object is to get its candidates into office.

In the elections for each house of Congress and for the White House, one of two results for a party is possible, win or lose. In Congress, all is not lost for the losing party: its financial rewards in the form of campaign contributions will be less than if it had won, but it will still score enough to fight another day. (But a loss is a loss.)

The trick is to expand the appeal of the party sufficiently to ensure that it sends a majority of members to each house, but not so broadly that it falls apart from ideological incoherence or sectional or personal rivalry.

Fortunately, the system has proved pretty robust in the past.

Thus, for three decades from 1932, the Dems were actively courting the Negro vote (which had previously been given to the GOP in homage to Honest Abe, rather than anything done for them after 1876) and, at the same time, providing a cosy home for segregationists, some with mouths as big and unpleasant as Theodore Bilbo's.

If Northern liberals in the 1930s were ashamed of not even being able to pass an antilynching bill (and I'm not clear how many of them were), that didn't for a second make them think of taking a principled decision to abandon their colleagues from Dixie, and thereby condemn themselves to eternal minority.

If a party can manage a straddle of such eye-watering width, it can surely accommodate both Bob Casey and Russ Feingold!

In fact, the range of ideology which a party needs to accommodate is rather determined by the electorate: there are not enough liberals to provide 50%+1 of either house, so moderates must be taken on board.

On the other hand, ideology is far from everything: a few days ago, I looked at the ideological makeup of the Senate in the 108th, as measured by DW-NOMINATE scores.

For states with either two Dems or two GOP senators, the range of scores was small. For states with one of each, the range was six times wider.

Since there are no longer any crossover senators (ie, there are no Dems with a more conservative rating than any GOP, or vice versa), any Dem is better than any GOP, ideology-wise.

If a state as red as Nebraska reelects Nelson (as it most likely will), it's clearly not voting its ideology. (And vice versa for the Maine sisters.) Other qualities in a candidate (or defects in his opponent) clearly count for a good deal, and incumbency probably counts for more. But there must be a limit to how liberal a candidate may be and still get elected by a conservative state.

(If, over the longer term, the Dems are looking to get back into the South in the Senate, it will probably be done with populist moderate-to-conservatives. Clearly, allowing the GOP the edge of their very own permanently Solid South would be a less than advisable Dem strategy.)

If lefties think they have a way to produce 218 Dem Congressmen and 50/51 Dem Senators without needing to go as far right as Casey, or Ben Nelson, or Landrieu or wherever else one might draw the line, let them demonstrate it.

I can't see it myself: but I'm always happy to receive an education.