It has often been easier to understand the Democratic Party as a series of shifting coalitions rather than a cohesive, disciplined unit. That long-term tolerance for turmoil may even help to explain how this party — or at least this party label — has stuck around so long. At two centuries plus, these Democrats are the oldest political party still functioning, not just in the U.S. but in the world.

The divisions have at times played out in marquee presidential races, as when the liberal Sen. Edward Kennedy of Massachusetts challenged the re-nomination of Jimmy Carter, the more centrist Georgia Democrat who happened to be the incumbent president in 1980.

More often, the tensions that permeate the party's past have been part of the ordinary, daily business of Congress. For generations it was understood that in Congress, the Southern Democrats would go their own way when they felt their regional or ideological interests were at stake.

So stark was this reality that for decades Congressional Quarterly tallied and reported votes in Congress for three parties: Republicans, Southern Democrats and National Democrats. When a majority of members in each of the first two categories agreed, CQ labeled it "the Conservative Coalition."

The problem for Biden and the Democrats of the current Congress is not that they are so fractious but that there are so few of them. In a 50-50 Senate, even one defector can deal a fatal blow. In a House where the majority's margin of error is three votes, the intraparty balancing act is a high-wire acrobatic trick of the first order.

Activists pressuring Biden to "go big" continually refer to the Democrats' "total control of Washington," when in reality there is no such thing. Given the narrow-to-nonexistent margins, the idea of control assumes every Democrat would vote in lockstep, and that has almost never been the case.

At those fleeting moments in history when the Democrats have united and thrived in Congress, they have done so by uniting urban and rural voters across the regional lines. But these interludes of cooperation, symbolized by alternating House Speakers from Texas and Massachusetts (the "Austin-Boston axis"), have always been highly pragmatic.

It was understood that one side would support the other in exchange for the protection of certain overriding concerns. For Southern Democrats, the paramount concern was segregation.

That bargain worked in the New Deal years of the 1930s, when Franklin D. Roosevelt enjoyed huge majorities in both House and Senate that had votes to spare. It was still the order of the day when Lyndon B. Johnson stepped into the presidency in place of the assassinated John F. Kennedy.