A democracy - especially a liberal democracy - is only as strong as its political institutions. This is a rare truism in political science, and a cause for serious concern among believers in the American democratic experiment.

Although it was clear from day one that U.S. democratic institutions would be stressed under President Trump, the extent of the erosion is remarkable. The incumbent Republican Party has gone all-in on an illiberal demagogue empowered by an increasingly untrammeled executive branch, with even impeachment swatted down like a bothersome institutional fly. Judicial and prosecutorial authorities are increasingly politicized, as are national security agencies, exacerbating threats to electoral integrity from foreign interference, pervasive misinformation and gerrymandering.

Meanwhile, federal institutions like the Electoral College and a rural-biased Senate interact with structural demographic forces - the end of the country's white Christian majority; a skills and educational gap between urban and rural areas - to heighten social polarization and erode the critical link between popular will and political outcomes.

In this context, the Democratic Party is the main institutional bulwark stemming further erosion of America's liberal democracy. Only the Democratic Party can prevent Trump's reelection in November, and only the Democratic Party can categorically defeat a GOP now tragically united by its tolerance for demagogy.

And until last week, the Democratic Party appeared to be collapsing on itself.

If the Democrat's record-setting field of 20+ presidential candidates, the stridency of their social-media warriors and the dysfunction of the Democratic National Committee's primary process didn't raise waving red flags about the weak and divided state of the party, the emergence of Sen. Bernie Sanders (I-Vt.) as frontrunner certainly did. Sanders, a self-described socialist rejected by the Democratic establishment as a carpet-bagging populist pretender, was clearly the most popular candidate with the party's activist base.

Meanwhile, longtime poll leader former Vice President Joe Biden - the epitome of that very same establishment - was too weak to consolidate even moderate support, never mind that of the national electorate. Biden's weakness invited the entry of billionaire Michael Bloomberg. The former Republican New York mayor's brief burst in the polls told us as much about Democratic institutional anemia and vulnerability to outsiders as the more sustained success of an independent like Sanders.

Clearly, the Democratic Party has proved a more robust institution than all that. Combined with primary voters overwhelming desire to defeat Trump and key support for Biden in the African-American community, the rise of Sanders in early primaries and fears of anti-socialist backlash down-ticket has prompted a consolidation of moderate support around the vice president. After Super Tuesday, the Democratic contest has become a clear two-person race between Biden and Sanders, with the former seemingly in the pole position to take on Trump come November. The Democratic Party appears to be fulfilling its core institutional function: Aggregating voter preferences and facilitating bargaining among party elites to coalesce around the strongest challenger to beat Trump in November.

Yet all is not well - not by far. Biden is still a weak candidate, prone to gaffes and vulnerable to both a reversion of voter apathy and undoubtedly harsh attacks by both Bernie's fervent base (re: his long and largely centrist record) and Trump's Republican allies (re: his son Hunter's business dealings). Bernie and his supporters still seem to believe that socio-economic revolution is a winning political message in the most culturally capitalist, religiously conservative country in the West. And Trump is still a very real threat to U.S. democracy.

Fortunately, there is a solution that will leave seasoned analysts of U.S. politics aghast but seems perfectly natural to a comparative political scientist like me. The Democrats must form a coalition. The presidential ticket should be Biden-Bernie, or even Bernie-Biden. And together they can defeat Trump and the Trumpist GOP.

Isn't Bernie a socialist with an unrealistic, left-wing policy agenda? Is Biden too emblematic of the party establishment? Perhaps so - but so what? Both agree that defeating Trump is the party's overwhelming priority. Biden is strongest with African-Americans and suburban whites; Bernie with younger voters, both white and Latino. A joint ticket enables Democrats to maximize turnout and spend the next six months negotiating unity rather than exacerbating disunity. This was true after Iowa, and it is still true today.

Prominent Bernie supporter and leftist superstar Rep. Alexandria Ocasio-Cortez (D-N.Y.) has already laid the groundwork for this political constellation, probably unintentionally. Ocasio-Cortez claimed that in another country - say, one governed by a parliamentary system - she and Biden would be in different political parties. That's true; and that's also the point. In a parliamentary system, moderate Democrats and Democratic socialists could, and almost certainly would, form an electoral coalition to displace Trump. The winner-take-all nature of the U.S. electoral system (including, somewhat uniquely, party primaries) mitigates against this outcome - but it does not have to. Given the overwhelming, mutually agreed prerogative of beating Trump, there is no reason why the Democrats should not unite in order to do so.

To be sure, U.S. presidents generally do not pursue a "team of rivals" approach to governance, especially before an election. And the bad blood between the party establishment and the Sanders camp is clear for all to see. But then the urgency of uniting to defeat Trump is even clearer.

Helpfully, this majority coalition already has a perfect name: The democrats. Just emphasize the lowercase "d."

Mark Y. Rosenberg is CEO of GeoQuant and an adjunct professor at Columbia University's School of International and Public Affairs.