Partisan enthusiasts look forward to their side achieving lasting majority status. Others might take counsel from the political scientist David Mayhew, who casts doubt on whether permanent or long-lasting majorities are possible. When you look closely at the supposedly permanent partisan majorities of the past, they fade from view.

Republicans won all but two presidential elections from 1860 to 1892. But Democrats won majorities in the House for most of that period after the Southern states were readmitted to the Union. Republicans won all but two presidential elections from 1896 to 1928. And they held congressional majorities for most of that time, as well. Yet they won almost nowhere in the South, and at the time their dominance was by no means taken for granted.

And what of the New Deal Democratic majority from 1932 to 1968? New Deal Democrats took a hit in the off-year elections of 1938, and polling suggests the Republicans would have won in 1940 if domestic issues had been paramount. Instead, voters re-elected Franklin Roosevelt as a wartime president in 1940 and 1944.

Harry Truman, too, benefited from a foreign issue -- the successful Berlin airlift -- in 1948, and John F. Kennedy campaigned in 1960 as the most determined of Cold Warriors. The Democrats held Congress during almost all this period. But as liberal historians note mournfully, liberal Democrats had effective majorities for only a couple of years from the 1930s to the 1960s.

All of which suggests to me that the more natural state of partisan politics, in this country at least, is something less like party dominance and more like uneasy equilibrium. Equilibrium that swings to one side or another from time to time, as it has swung in varying measure to Democrats in 1992 and 2008 and to Republicans in 1994 and 2004.

Because of their basic character, both parties have difficult tasks in assembling and holding together majorities -- Republicans, because their core constituency is off-putting to those whom it defines as something other than typical Americans; Democrats, because of the difficult of holding together what is usually a very diverse and conflict-prone coalition.

Barack Obama now has that task. He has shown unusual skills and the capacity and willingness to stress what he has in common with those on the other side of the partisan divide. But already rents are appearing in the Democratic fabric -- over Rod Blagojevich, same-sex marriage and the unions' card check bill. My guess is that Obama will hold his majority together for a good long while, but not forever.