Leftists in the USA do not add up to an organized Left. We are very much like the peasants of France whom Marx criticized in "The Eighteenth Brumaire of Louis Napoleon": "potatoes in a sack." Potatoes in a sack can't hope to seize any political opportunity, whether to contest or cooperate with the Democratic Party.
If there were an organized Left in the USA, even as electorally small as the UK SWP and the French LCR*, then, we could talk about which strategy would make more sense at a given time in the USA.
Even if there were an organized Left in the USA, however, the position like the LCR's, which I believe is appropriate for the French Left, is probably untenable in the USA, given that Washington is the hegemon of the multinational empire whereas Paris isn't.
* <http://mrzine.monthlyreview.org/besancenot230407.html> Olivier Besancenot Speaks
[...]
...On 6 May, we will be on the side of those who want to prevent Nicolas Sarkozy from attaining the presidency of the republic. It is not a matter of supporting Ségolène Royal but voting against Nicolas Sarkozy.
[...] ===================================== It depends what you mean by "the US left". I suggested previously that it should be taken to include all who broadly suppport the demands of the unions and social movements at home and are opposed to foreign intervention abroad - people who would variously describe themselves as left liberals, social democrats, Marxists, or anarchists.
The American left is overwhelmingly left liberal unlike elsewhere where is strongly social democratic, although there is now little to distinguish between them except for their historic origins. On the far left, Marxists in most countries have seen their parties atrophy to the point they are no longer viable political alternatives with mass followings. They are propaganda groups like the LCR, which are invariably forced to fall in behind the mass social democratic parties against the parties to their right, notwithstanding their convoluted disclaimers in doing so, which are mainly variations on Lenin's original theme of "the rope supporting a hanged man". The anarchists scorn electoral politics as they prepare for mass insurrection.
American left liberals may not be as politically coherent or consistent, but they are by far the largest component of the broad US left and call the Democratic party their political home. Thanks to Iraq and the other follies of the Bush administration, they are angry and energetic, and have helped move US mass politics in a healthier direction than at time under the Bush administration. This is something you and other US leftists usually omit from your analyses.
Julio Huato, who is active within this sizeable constituency and knows it well, might want to comment further, but then again he might not. The DP discussion, as you say, is largely idle chatter on these lists because of the weakness of the far left, and it often turns ugly because of the frustration arising from that condition.