The Democratic Party is a center-left party. At this time, it is the biggest tent. The Democratic Party is not a centrist party, but it has the greatest room and tolerance for centrists.
The left wing of the Democratic Party was been disappointed with President Obama. They have mounted primary challenges to several centrist Democrats.
I believe the big advantage that centrists have in the Democratic Party, as opposed to the Republican Party, is that these attempted purges have not, for the most part, succeeded. Several establishment Republicans have been knocked off by the Tea Party wing. No establishment Democrats have been knocked off by leftists in the Democratic Party this cycle. (I don't think anyone could count Senator Spector as an establishment Democrat.)
After President Reagan's defeat of President Carter in 1980, the Democratic Party was torn apart for a season by ideological fights and recriminations. The metaphor of a circular firing squad was appropriate. The party was brought back by a rising generation of centrists who were willing to horse trade with the other side. The country, and the world, enjoyed a moment of peace and prosperity.
The Republican Party is having its circular firing squad moment now. The emotional energy is on the right wing. But the future of the party lies, I believe, with a rising generation of centrists who will be willing to horse trade with the other side.
This is a great moment for the Democratic Party. I believe the Obama administration has done about as good a job as could be done in cleaning up the massive destruction they inherited, of which the Gulf oil spill is only the latest legacy. At the same time, they have had a few significant legislative and diplomatic achievements, with more to come before the mid-term elections. The party in power will, no doubt, lose seats in the mid-term, as usually happens. But eventually there will be centrists Republicans to work with, who will strengthen the centrist Democrats. Together they can use America's moment as the world's super power for the good of all.