How could Roy Moore win? Because parties are weak and partisanship is strong.

Alabama might elect a senator whose past includes dating teenagers when he was in his 30s, and getting kicked out of a mall for predatory behavior. How could this happen?

The same way Donald Trump happened. The most important feature of our political age is that parties are weak while partisanship is strong.

The Republican Party, in particular, has lost control over its primaries: Like Trump before him, Moore was opposed by the GOP establishment but won the party’s nomination anyway. And having won his party’s nomination, he is now buoyed by the power of modern partisanship: Alabama is overwhelmingly Republican, and Republicans do not want to vote for a Democrat, or let one win.

It’s worth dwelling on how different, and how surprising, this era is. As a political scientist, I can tell you that the conventional political wisdom of the 1980s and 1990s subscribed to a candidate-centered politics, in which an increasingly independent electorate would “vote for the person, not the party.” Parties, those old dinosaur organizations, would soon fade from view, we were led to believe. And partisanship would go with them.

The last two decades have illustrated that this prediction was disastrously wrong — or at least half wrong. Parties have indeed lost influence and authority, showing little ability to beat back problematic nominees. But partisanship is stronger than ever.

The Alabama case illustrates several facets of party weakness. During the primary, Moore was decidedly not the choice of the Republican Party establishment, which backed his opponent Luther Strange. Even Trump weighed in, tweeting that Strange “has my complete and total endorsement” back in August. Still, Moore won a solid primary victory, beating Strange by almost 10 points.

After the primary, the state party’s hands were formally tied. Alabama law prevents the replacement of a candidate on the ballot within 76 days of the election. Once the reports of Moore’s predatory behavior began surfacing, the Alabama Republican Party had few options for removing him from the ticket. Thus, local Republican elites have to choose between supporting Moore or conceding the race to a Democrat, and for the most part, it looks like they’re sticking with Moore.

While Moore dropped in the polls after the allegations were revealed, partisanship has certainly shaped his remaining support. A recent poll found that 71 percent of Alabama Republicans think the news reports about the Senate candidate are false. In an interview with CNN, one Alabama voter said, “I don’t even believe the allegations. There’s lots of fake news going around these days.” Another told the New York Times, “Until there’s more concrete evidence, I would err on the side of the Republican, strictly for tax reform.”

The reaction of Alabama Republicans illustrates several dimensions of partisanship now. These include a preference for issue stances over candidate characteristics, partisan reasoning behind how they interpret new information, and deep distrust of the motives of their political opponents.

Behind the power of modern partisanship is what researchers call “negative partisanship.” The Georgetown government professor Jonathan Ladd defines negative partisanship as “the tendency to vote for a party not mainly because you like it, but because you are repulsed by the other major party.”

Research by University of Maryland political scientist Lilliana Mason suggests that Americans have become “angrier toward their electoral opponents, and more proud of their own candidates, since the 1980s.”

Recent polls illustrate this anger and distrust. Partisans report high levels of anger and distrust across party lines, and tie the reasons for their own partisanship to a belief that the other party’s policies are dangerous and bad for their country.

It’s tempting to write these differences off as superficial: Indeed, some researchers have argued that many Americans actually share many beliefs. But while there may be some truth to these arguments — most people aren’t, after all, terribly attentive to politics or political ideologies — they seem increasingly inadequate to the present moment.

Instead, more recent research illustrates how, in the words of political scientists Steven Webster and Alan Abramowitz, “Americans have sincere ideological disagreements about policy.” In other words, Alabama Republicans’ dislike of Doug Jones isn’t just about animosity on the surface or competition between two teams. It’s the result of truly distinct beliefs about policy issues, and perhaps in that regard the reluctance to cross party lines is rational.

But strong negative partisanship isn’t just about issue positions. After all, Trump was hardly a perfect spokesman for conservative ideology during the 2016 race. Luther Strange’s issue stances are probably better aligned with mainstream conservative Republicans. Once a party nominates its candidate, loyalty — and aversion to the opposite party — seems to kick in. In both cases, this seems to be especially true with regard to the issue of abortion.

Moore’s trajectory mirrors what happened with Trump during the 2016 election. For all the chaos, Republicans voted Republican, and Democrats, for the most part, voted Democratic. Exit polls showed around ninety percent of both Republicans and Democrats voting for their party’s presidential candidate in 2016. The Alabama Senate race has also thus far shown how strong these forces can be.

National resistance to Moore has been greater than it was to Trump — both House Speaker Paul Ryan and Senate Majority Leader Mitch McConnell have stated that Moore should step aside. And Alabama’s senior senator, Richard Shelby, has said that he cast an early ballot for someone other than Moore.

But last week, President Trump tweeted that the party needed Moore to win because “We need his vote on stopping crime, illegal immigration, Border Wall, Military, Pro Life, V.A., Judges 2nd Amendment and more.” And the RNC contributed financial support to Moore’s campaign.

Trump’s message seems well designed to activate negative feelings toward Democrats, and to remind Republicans of hot-button issues separating the two parties.

Support among elected Republican officials in Alabama has been consistent, if not always glowing: a New York Times article from November quoted local party leaders pointing to the principle of “innocent until proven guilty,” and predicting that, “If the party had gone against our candidate, I do think a lot of people would have lost faith in the party and possibly in the whole political process.”

If Moore wins, the reason will be simple: He won the Republican primary, he is running in a heavily Republican state, and Republican voters want to be represented by a Republican Senator. Even if that senator is Roy Moore.

Julia Azari is associate professor of political science at Marquette University. She is the author of Delivering the People’s Message: The Changing Politics of the Presidential Mandate, and she blogs at Mischiefs of Faction.

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