Several factors — the fragmented media market, super PACs, new media — have made it easier for factions both within and outside the party to coordinate on their own. The Tea Party and the progressive movement can empower candidates like Rand Paul, Ted Cruz and Bernie Sanders. Campaign finance laws have undermined parties and empowered individual candidates: Most think Newt Gingrich and Rick Santorum lasted longer in 2012 than they would have without super PACs.

In several contests, party leaders were hesitant to challenge candidates who appeared to have independent support. In the 2004 Democratic contest, most party leaders did not want Howard Dean as the nominee, but his fund-raising was impressive. It took Mr. Dean’s failure in the Iowa caucuses to embolden party leaders to back John Kerry.

That the evolving political environment would give the party leaders trouble is no surprise. The invisible primary itself evolved as a response to the 1970s reforms, but it took party leaders a few cycles to work it out. They may yet work out these changes as well.

Maybe the party is falling apart. Even without those obstacles, coordination was going to be especially difficult for Republican leaders in 2016.

To see why, look no further than the House of Representatives. In October, House Republicans struggled to find a speaker who could win the support of the conservative House Freedom Caucus as well as more mainstream party members. That same cleavage is now doing for the presidential nomination what it did for the routine in Congress.

We argued in the book that you should think of the party as a collection of different, often competing groups that nevertheless find ways to bridge their differences. Right now, the largest divide is between those groups that some call “party regulars,” or the establishment, and more ideological groups — the Tea Party, religious conservatives and other movement conservatives.

Jeb Bush was a clear early choice among many regulars, but he was just as clearly not the choice of movement conservatives. And Mr. Cruz, rejected by the national party, has been embraced by many movement conservatives. Before voting began, Mr. Cruz led in endorsements from politicians at the state level, where the Tea Party movement has been most effective.