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What do parties do? Donald Trump's candidacy is a good test case.

Joe Raedle/Getty Images

So far, Donald Trump has continued to be a real pain for political science. He didn't lose interest or get winnowed out of the race. His inflammatory remarks — tweets about Ted Cruz's wife, controversial comments about abortion — haven't ended his campaign. And although he may well lose in Wisconsin this week, part of the cycle of wild Trump news stories is the string of primary victories he has pulled off time and again.

Buried under the layers of shock, however, is a more substantive lesson about politics: As the first real political outsider to permeate the usual party processes over the objections of party elites, Trump provides a useful test case for what parties really do in our system. As Hans Noel wrote back in September, "Political science wins when things go haywire, because we rarely get to observe the counterfactuals that we base so much of our reasoning on." It's not every election that we get to see this counterfactual: What does democracy outside parties really look like?

You could object to this characterization on the grounds that Trump has, in a way, established himself as a party figure — he's gained endorsements and, of course, primary votes. But it's clear that unlike other candidates who embraced the outsider image while appealing to party leaders — Ronald Reagan, Barack Obama — Trump is something else.

The party's halting and controversial embrace of his candidacy tells us something about the ubiquity of parties and about the limits of their capacity to shape events. But if we treat Trump as primarily a candidate unfettered by party, his candidacy presents the closest thing to a "natural experiment" that we are likely to see — and it's a good test of our theories about what parties are and what they do.

First, parties provide core principles that constrain the positions candidates can take. Many of the recent objections to Trump from mainstream Republicans — often accompanied by Cruz endorsements — have focused on Trump's inattention to conservative principles. This is a really important point of distinction between two candidates who otherwise have defied the preferences of the party establishment.

We don't often think of parties this way, but functional organizations tend to have lots of people who are competing and training for leadership spots. A purely candidate-centered movement like Trump's doesn't work like this. A good contrast case is the Tea Party, an organizationally complex movement with lots of politicians vying to lead it. (You could probably argue that the Tea Party was too successful in this regard, recruiting too many potential presidential candidates who got in each other's way.)

Trump isn't constrained by any such competition from others; his campaign is all about him. The implications of this aren't entirely clear yet — but one possibility is that for supporters drawn to his policy positions and what he represents, he is the only option. This probably doesn't mean a lot of staying power for the movement, but it also means there's very little capacity to check his behavior.

Similarly, parties also provide consistency and expertise. It's not uncommon for presidential candidates to have some gaps in their preparation for office, sometimes some pretty serious ones. Parties supply advisers who can link new candidates with the party's past and priorities. When George W. Bush ran in 2000, his own statements about turning away from foreign involvement were belied by his selection of Dick Cheney as vice president and Donald Rumsfeld as a foreign policy adviser on the campaign and eventually as secretary of defense.

These things cut both ways, of course. The movement Cruz is a part of has been criticized for being too ideologically rigid and inflexible. The Bush foreign policy team, heavily steeped in neoconservative orthodoxy, was criticized for its failure to draw on a more diverse set of perspectives — for its exclusion of dissenting views. Viewed this way, parties play a complex and often ambivalent role in shaping what presidential candidates and presidents do.

The advantage of diverse coalitions is that they can theoretically represent broad ranges of views, keeping a powerful office from becoming too narrow or autocratic. At the same time, as ideology has increasingly become a key part of coalition building and party "brand," enforcing a narrower range of acceptable party beliefs seems to have become part of what parties do.

This election cycle has also taught us some things about what parties are not. The diffuse nature of party membership has long been of note to scholars. What does it mean for a citizen — or a politician — to call themselves a Republican or a Democrat in the United States? Trump's candidacy (and Bernie Sanders's, for that matter) suggests that these labels can be especially loose at the elite level.

The mass membership of parties is hard to define, too. Barry Burden and Jordan Hsu show that the Republican primary electorate has included quite a few independents this time around.

What has long characterized American parties instead is the rules they follow. These rules have allowed parties that were ideologically and geographically diverse to settle, most of the time, on presidential nominees every four years. Trump's use of the party's formal nomination rules — primaries and delegate allocations rather than endorsements and informal winnowing — illustrates that the formal rules of the game are still a great deal of what defines a party. And those rules have been revised to give party leaders relatively little formal voice in the outcomes of the nomination process.

If Trump wins the nomination, the general election will provide an even more extensive test case. The extent to which his campaign shows restraint toward the Democratic nominee, the consistency of his issue positions, and the development of his policy team will all give us some insight into what a presidential candidate without a party might be. And there's a chance that the differences at that point will be minimal. But the rules will tell us how he got there. That alone will be a major lesson in what parties do.

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