As Biden voters celebrated in the streets this past weekend, Democratic politicians began to fight among themselves. Although they won the presidency, Democrats lost seats in the House and failed to flip a single state legislative chamber. What went wrong in down-ballot races?

That was the subject when House Democrats held a three-hour post-mortem conference call last week. Agitated moderates argued that Republicans easily aimed debilitating attacks at left-leaning positions on defunding the police, Medicare-for-all and socialism. Rep. James E. Clyburn (S.C.) warned that the party should tone it down to win the Georgia runoffs and take control of the Senate.

From the left, Rep. Alexandria Ocasio-Cortez (N.Y.) responded by arguing that the problem was operational, not ideological. “The party — in and of itself — does not have the core competencies” needed to run campaigns, she insisted in an interview. She argued that too many campaigns, following the lead of the Democratic Congressional Campaign Committee (DCCC), failed to build a strong online presence or to invest aggressively in volunteer recruitment, get-out-the-vote drives, and person-to-person persuasion.

These intraparty conflicts are rooted in the party’s relationships to social movements, and will continue. But our research suggests Ocasio-Cortez put her finger on a crucial difficulty for Democrats. They have historically been ambivalent about what political scientists call “party building — creating lasting organizational structures that would allow partisans to organize. Past Democratic presidents — including Barack Obama — have let grass-roots Democrats’ energy and activism dissipate when they came to power. Now we are going to see if this pattern reasserts itself when a new president prepares to take office.

American political parties have been hollowed out

As Americans have polarized, both the Democratic and Republican parties have become ever more central to politics. However, they have simultaneously lost their capacity to organize an effective collective response to social challenges. Democrats in particular have historically paid sporadic and superficial attention to the mechanics of voter engagement and organizational investment. In consequence, the party has become hollow — top-heavy at the national level, weak at the state and local levels, and lacking a rooted, tangible presence in the lives of voters and engaged activists alike.

How did that happen? Political parties suffered particularly from the atrophy of civic life in the later 20th century. Patronage-oriented political machines and volunteer-led, issue-oriented party clubs alike fell on hard times. Their successors have found it far harder to build party institutions rooted in local communities.

Today’s polarized parties are less organizations than they are networks, relying on a bevy of blob-like outside entities. These include issue groups, media and consultants. They are linked to the parties but not formally part of them, and they are all looking for a share of the money sloshing through the system.

In the new, post-Citizens United world of campaign finance, megadonors have increasingly opted to fund independent expenditures unencumbered by the rules that constrain formal parties. The “super PAC” counterpart to the DCCC, for example, is the House Majority PAC, which made significant independent expenditures and performed other party-like functions outside the formal party structure, with overall electoral results that still disappointed.

Democratic presidents have hurt their parties, not helped them

If any individual can bring order to these unwieldy components, it is the president. Yet paradoxically, the Democratic Party has suffered the most under Democratic presidents. Republican presidents going back to Eisenhower have systematically invested in their party’s organizational capacities at the national, state and local levels: funding local party-building initiatives, assiduously recruiting activists, volunteers, and candidates, teaching campaign techniques, and launching fundraising systems. Democratic presidents, in contrast, have repeatedly emphasized enacting policies over party-building.

As Ocasio-Cortez correctly pointed out, Obama failed to sustain the momentum of his groundbreaking 2008 and 2012 campaigns. He diverted the best talent, data, and analytics into Organizing for America, an outside 501(c)(4) group legally prohibited from coordinating with the Democratic Party. The Democratic National Committee (DNC), for its part, abandoned Howard Dean’s much-admired 50-state strategy. When Hillary Clinton inherited the DNC in 2016, she found it “on the verge of insolvency,” riddled with bad data and riven with internal strife. Before leaving office, Obama acknowledged he failed as a party-builder: “We did not begin what I think needs to happen over the long haul, and that is rebuild the Democratic Party at the ground level.”

Things are changing — a little

There are some signs of revival. In Nevada, former senator Harry M. Reid professionalized and grew the state party as the central institution in the state’s Democratic politics. The Nevada party’s large permanent staff includes full-time positions in areas like field work and opposition research. That enables party leaders to operate with longer time horizons than the next Election Day and to avoid the coordination problems endemic to party networks. Nationwide, meanwhile, the Trump-era “Resistance” has revitalized civic engagement. Those outraged by Trump’s election — a group dominated demographically by middle-aged-and-older college-educated White women — have turned sleepy Democratic county offices across the country into hotbeds of activity.

Yet this has limits. Yes, Joe Biden prevailed in Nevada despite weakness among Hispanic voters nationwide, and Resistance activists succeeded in getting Democrats elected from the suburbs. But Democrats failed to expand their victories beyond the high-water mark of 2018. Nor did they win down-ballot elections, from state legislative seats to the Senate, which activists had been working for years achieve. Biden ran ahead of House Democrats.

In other words, activists’ multiyear mobilization never fully made the leap from anti-Trump campaign to full-scale party endeavor.

The more Democratic factions squabble, the less they will cooperate on party-building. Yet Democrats face a daunting core challenge in winning majorities: Their supporters skew urban in a political system that advantages rural voters. As politics becomes nationalized, it is ever harder for state and local parties to separate themselves from the party’s overall brand. We are about to discover whether the incoming president — the person best positioned to rebuild a party from top to bottom — will devote the attention and resources that party building would require.

Correction: An earlier version of this article asserted that the 2020 electionefforts of the House Majority PAC, led this cycle by Hillary Clinton’sformer campaign manager, Robby Mook, seemingly had little electoraleffect. Because no research data is available to evaluate the effortsof Mook or the House Majority PAC, that assertion has been removed andthe story has been updated.

Daniel J. Galvin (@Daniel_J_Galvin) is associate professor of political science and faculty fellow at the Institute for Policy Research at Northwestern University and author of “Presidential Party Building: Dwight D. Eisenhower to George W. Bush” (Princeton University Press, 2010).

Daniel Schlozman (@daschloz) is Joseph and Bertha Bernstein Associate Professor of Political Science at Johns Hopkins University and author of “When Movements Anchor Parties: Electoral Alignments in American History” (Princeton University Press, 2015).

Sam Rosenfeld (@sam_rosenfeld) is assistant professor of political science at Colgate University and author of “The Polarizers: Postwar Architects of Our Partisan Era” (University of Chicago Press, 2018).