Despite being an outsider with limited commitment to Republican ideology, Donald Trump won the weakened party's 2016 presidential nomination. (Evan Vucci/AP)

There’s a paradox at the heart of American politics: Political polarization has rarely been more extreme. More than half of registered Democrats and Republicans view members of the other party “very unfavorably,” and 49 percent of Republicans (and 33 percent of Democrats) would prefer their children not marry a member of the other party (up from about 5 percent in the 1960s). On a practical level, the 2018 midterms produced a deeply divided Congress unlikely to generate much legislation.

But none of this is because parties are “unusually strong,” as Bloomberg’s Jonathan Bernstein and other commentators put it. On the contrary, our political system is dysfunctional and polarization is intense because parties are too weak.

A “strong” party is a party that presents voters with a coherent policy agenda. To be able to construct and deliver on a party platform, party leaders select candidates; but backbenchers in the legislature in turn choose and replace party leaders depending on how successfully the leaders deliver electoral victory.

A strong Republican Party would never have allowed Donald Trump, an outsider with limited commitment to Republican ideology, to become its presidential nominee. It would not have allowed Roy Moore to have been its nominee for the U.S. Senate from Alabama in 2017. A strong party would not tolerate a rebellious faction like the tea party.

On the Democratic side, it’s unlikely a strong party would have selected the democratic socialist Alexandria Ocasio-Cortez as the nominee for New York’s 14th Congressional District. When parties are strong, they are more likely to pursue national policies that appeal to the median voter — and they will select local candidates with that in mind. To win in single-member districts, the party has to be strategically moderate.

U.S. parties used to be somewhat stronger, with party leaders playing a role in selecting candidates all the way up to the presidency. A key turning point was 1968, when Vice President Hubert Humphrey received the Democratic nomination for president despite not having run in any primaries, and despite his support for the Vietnam War. Grass-roots Democrats were outraged. A commission co-led by George McGovern, the future presidential candidate, recommended changes to create “popular control of the Democratic Party.” Primaries would be the central way of selecting a candidate, and women, black Democrats and young people would receive quota-like representation at the party convention.

But these small-d democratic reforms have produced less government responsiveness, not more. Primaries are a chief culprit: Relatively extreme voters turn out in higher numbers than the typical voter for their party. We have had primaries in Congress since the Progressive era, but they have become much more important because of red state/blue state sorting, partisan and bipartisan gerrymandering, and the advent of majority-minority districting, which have combined to vastly increase the number of safe seats. This often makes the primary the only meaningful election. These primary voters then choose representatives who resist moderate positions — representatives who will not, in turn, choose strong congressional leaders capable of enforcing moderation.

The weakness of parties exacerbates all sorts of problems. Many critics point to the role of money in politics as the source of America’s political ills. But that gets the causal arrows of the problem the wrong way around. Politicians need staggering amounts of money to pay for highly individualized campaigns. If they could campaign on a coherent party platform of broadly popular policies, their election costs would be far lower — as they are in countries with strong political parties such as Britain and Germany. In those countries, people tend to vote for the party, not the individual. “I would vote for a pig if it was my party’s pig,” as the British saying goes.

Frustrated by ineffective government, voters in California and elsewhere have sought to take matters into their own hands through ballot initiatives, yet another democratic-sounding innovation that undermines democracy — and undermines parties, too. The trouble with this kind of DIY politics is that, by tackling one policy at a time, voters forgo a major value of political parties: the ability to campaign on and implement a full suite of policies that serve the interests of most people over the long term. Proposition 13 in California catered to people who cared intensely about tax cuts in 1978, but only by ignoring the downstream effects on California’s schools and local government. (Brexit, the result of a British referendum, was similarly shortsighted, and it similarly undermined the party system.)

Political parties in the United States are weak by design. The Founders built a political structure of multiple checks and balances to thwart tyranny — at the cost of governability. When Jefferson and Madison belatedly embraced parties in 1800, they had to run a formidable obstacle course of their own making: the ability of a president and of states to thwart partisan unity in Congress around policies and programs. Parties today face similar challenges but have hamstrung themselves with developments like primaries.

How can we get closer to a system of strong, competitive parties, given our constitutional structure?

A popular, supposedly democracy-promoting reform these days is abolishing the electoral college. This is barking up the wrong tree. Yes, a direct, nationwide presidential election would diminish the disproportionate influence of rural voters on the presidential race, but it would also enhance the power of the president, further undermining legislative parties.

There’s nothing about primaries in the Constitution. We should abolish them. Giving party leaders control over candidate selection conjures up “smoke-filled rooms,” but this is a red herring. Accountability is the foundation of strong parties, and party backbenchers themselves should empower leaders who can deliver winning platforms. It is inconceivable that a British leader who oversaw losses of the sort Nancy Pelosi did in 2010, 2012, 2014 and 2016 would keep her job. Controlling huge streams of campaign money (which we argue is a side effect of a weak party platform) is part of what gives Pelosi a grip on the party despite her ineffectiveness. The power of leaders should be conditional on their ability to choose candidates who can both win in their districts and support a coherent national platform.

Yes, the Democrats’ Ocasio-Cortez won her district as well as her primary. But she was the exception, because of the composition of her district. Progressive insurgents from more heterogeneous districts won their primaries but bit the dust Nov. 6: Kara Eastman in Omaha; Randy Bryce in southern Wisconsin; and Dana Balter in Syracuse, N.Y. A candidate-selection process that gave a stronger weight to the party would probably have chosen different candidates.

Short of abolishing primaries, a step in the right direction would be to count only the results of primaries with a turnout of 75 percent or higher, relative to the most recent general election. If that threshold were not met, the results should be thrown out and party leaders would choose an appropriate candidate.

Reforming the primary system should go hand in hand with the elimination of gerrymandering. The more each district represents the median American voter — the less it is carved to favor one party or another — the more parties will be required to compete for that voter.

We realize that calling for stronger parties sounds strange. Parties are at each other’s throats; why would we want to strengthen these monsters? But as we’ve shown, party weakness is to blame for polarization.

Strong parties capable of delivering on competitive policy platforms are the foundation of democratic accountability. We should strengthen ours.