To understand the difficulty President Biden is having enacting his agenda, keep in mind a couple of simple numbers: four and zero.

Specifically, there are effectively four political parties in Washington right now. And there is zero trust among them.

That alignment creates an awfully rocky road, on which Mr. Biden is stumbling as he tries...

To understand the difficulty President Biden is having enacting his agenda, keep in mind a couple of simple numbers: four and zero.

Specifically, there are effectively four political parties in Washington right now. And there is zero trust among them.

That alignment creates an awfully rocky road, on which Mr. Biden is stumbling as he tries to get his big domestic agenda passed. It would be an exaggeration to say Washington is ungovernable under these conditions, but at the moment it more resembles a parliamentary system with power divided among multiple weak parties than the traditional American system of two strong parties straddling the spectrum.

The Democrats who (barely) control Congress will try this week to regroup and finally agree on a $1.85 trillion social-spending and climate bill, after multiple failed efforts. That agreement might—only might—allow for the House to pass a $1 trillion, bipartisan infrastructure plan that has been languishing there despite Mr. Biden’s repeated pleas to his own party to pass the darn thing.

This impasse exists because the Democratic Party today really is two parties: the progressive version of the party and the moderate version. This split is hardly new, but now the two sides are roughly equal in congressional strength and leverage. Rather than bringing the two sides together, this balance of power has laid bare their differing agendas and priorities and driven them apart, and Mr. Biden has failed to bridge the gap.

This split is mirrored on the other side of the aisle, where the Republican Party also effectively is split in two. There remains the traditionally conservative GOP, which many Republicans refer to as the “governing part of the party,” because it is interested in advancing its agenda through conventional governing channels. Its power now is at least offset, and probably eclipsed, by the populist, nationalist version of the Republican Party, which is animated more by cultural fights with the left than by traditional conservative policy goals. This part of the party often seems more interested in blowing up the system than working within it.

Left to their own devices, the moderate version of the Democratic Party and the governing part of the Republican Party would fight out big tax and spending questions, and perhaps come to some workable compromises. And indeed, that is what they did earlier this year on the bipartisan infrastructure plan.

But moderate Democrats and governing Republicans aren’t really in charge any more, and they can’t work their will when outflanked by progressives on the left and new-wave populists on the right. As a prime example of that conundrum, consider that House Democratic leaders have twice scheduled votes on that bipartisan infrastructure plan, and twice had to beat an embarrassing retreat because progressives simply wouldn’t line up.

The divides are made harder to bridge because there not only is mistrust between the two parties, but now an almost equal measure of mistrust between the two factions within each party.

Among the Democrats, neither moderates nor progressives last week would accept Mr. Biden’s insistence that he had forged an agreement in principle on his social-spending and climate package, because neither side trusted the other not to change the deal at the last minute, before it was codified. And for good reason: Moderates were still trying to push down the overall price tag, and progressives were trying to jam back in health spending measures, even after Mr. Biden had risked his reputation by going on national TV to urge acceptance of the middle-ground compromise he proposed.

This is the kind of mutual suspicion usually reserved for negotiations between parties, not within a party. As if to underscore the point, liberal Sen. Bernie Sanders, who considers himself as much a socialist independent as a Democrat, went so far as to write an op-ed for a West Virginia newspaper two weeks ago specifically to build pressure back home on a fellow Democrat, Sen. Joe Manchin, to force him to fall in line behind more social spending.

President Biden discussed a $1.85 trillion social-spending and climate framework, calling the legislation "historic." Progressives have said they would block the infrastructure bill without a deal on the social-policy and climate bill. The Wall Street Journal Interactive Edition

The splits are reflected in the different ways Democrats describe their goals in passing the Biden agenda. The president, articulating the moderates’ views, describes a need to “invest in our people” to better compete in the great global struggle with China. Progressives describe a need to compel the wealthy and corporations to pay to address longstanding social inequities.

Among Republicans, the internal schism is illustrated by the frequent verbal attacks from the Trump/populist wing on “RINOs”—that is, alleged Republicans In Name Only. RINOs, of course, think they are the legitimate Republicans, not the populists the usurpers.

Amid these tensions, Republican alignments are shifting, as illustrated by a speech Sen. Marco Rubio prepared for a conservative conference this week attacking big corporations, the most traditional of GOP constituencies, as captives of woke culture.

Divided parties, declining trust: it isn’t a formula for an easy political life.

Write to Gerald F. Seib at jerry.seib@wsj.com