As we approach the change from a Republican to a Democratic administration, I have been thinking about the differences in the basic character of our two historic parties -- the oldest and third oldest free political parties in the world (number two, at least by my count, is the British Conservative Party).

Democrats are now hoping that their party can achieve something like permanent majority status. They can take heart that their presidential candidate won by a wider margin and their party has larger congressional majorities than the Republicans had when they entertained similar hopes four years ago. But there is reason for caution, and not just because the Republicans fell so far short. And the reason lies in the difference in the basic character of the parties.

The Republican Party throughout our history has been a party whose core constituency has been those who are considered, by themselves and by others, to be typical Americans. In the 19th century, that meant white Northern Protestants. Today, it means white married Christians. Yet such people, however typical, have never made up a majority in our culturally and regionally diverse nation.

The Republican core constituency tends to be cohesive and coherent (though sometimes, like now, quarrelsome). But it has almost never been by itself enough to win. As some Democrats like to remind you, Republicans have lost the popular vote for president in four of the last five elections.

The Democratic Party throughout our history has been the party whose core constituencies have been those who are considered, by themselves and by others, to be something other than typical Americans. In the 19th century, that meant white Southerners and big city Catholics. Today, it means blacks and singles and seculars and those with postgraduate degrees. Such people, while atypical, potentially make up a majority. But they often do not have a lot in common -- and when they have differences over highly visible political issues, they are hard to hold together.

As some Republicans like to remind you, Democrats have lost seven of the 11 presidential elections since their landslide victory in 1964.

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.