In speaking with Democratic organizers and operatives, it's been suggested to me that the Democratic Party of 2019 will resemble the Republican Party of 2015—a huge field of well-funded presidential candidates, and a party lacking any mechanism to effectively cull the field prior to the caucuses and primaries.
If this is true, it suggests a potentially serious problem for Democrats. Of course the caucuses and primaries will manage to eventually boil down the candidate field to one nominee. That part of the process still functions. But a very crowded field means the party might not end up with the sort of candidate it usually seeks. It could be forced to put forth a less experienced candidate, or a celebrity, or someone not particularly well versed in or committed to its policy stances. (See: the Republican Party in 2015.)
A crowded field could also enable a candidate to secure the nomination with only a plurality of the caucus and primary vote, as Donald Trump did in 2016. This would create a recipe for division and legitimacy problems within a Democratic Party that already suffered a good deal of both the last time around.
On the other hand, it would be unwise to simply ignore recent evidence that suggests Democrats are indeed better positioned than their Republican contemporaries—namely, what happened in California. Two weeks ago, Democrats looked with dread to California's top-two primary elections, worrying that a glut of Democratic candidates would split the vote and hand the two runoff slots to Republicans in several Democratic-leaning congressional and state legislative districts.
That didn't happen. Thanks to funding by the Democratic Congressional Campaign Committee, California Democratic party endorsements, and the support and coordination among dozens of prominent party leaders (both official and unofficial), the field of candidates was eventually culled. Some dropped out of the races shortly before the election, while others had a hard time maintaining support. Primary voters got the message, and Democratic lockouts were averted.
As Nate Silver noted, recent sub-presidential Democratic primaries suggest that the parties still do decide (most of the time, anyway) who receives the nomination, with the endorsements of prominent Democrats seeming to guide primary voters. Judging from this information, it's possible the Democratic Party is still functioning roughly as it has in the past, adapting to new impediments but still shaping the field of candidates and picking favorites.
Interestingly, though, that's roughly where the Republican Party was until recently. Back in early 2016, I noted at FiveThirtyEight that the GOP's fragmented and bizarre presidential nomination contest didn't seem to be trickling down to congressional races. Yet as recent research by Sarah Treul and Rachel Porter at Mischiefs of Faction notes, there's been a substantial change in who runs for Congress on the Republican side since 2016. Notably, the percentage of candidates with no government service at all has skyrocketed.
It's important to remember that the presidency, while an important office, is far from the only one in the American governmental system, and a party's efforts are not solely focused on the White House when deciding who should service in office. But events at the presidential level have a way of trickling down to other offices.
The faction of the GOP that backed Trump in 2015–16 was hardly the dominant segment within the party at that time. The "establishment" (to the extent that term makes sense) alternately didn't take Trump seriously or considered him a threat to the party's long-term success. But the establishment was also split between a variety of other candidates and never coordinated behind an alternative to Trump.
It's difficult to overstate the effect Trump's surprise victory in November of 2016 had on activists in the Republican Party, and possibly within the Democratic Party as well. It suggested that concerns about electability were overstated, that it might be OK to nominate a divisive rabble rouser without any of the assumed penalties at the ballot box. Republicans have responded to this message by signaling a tolerance for outsider candidates more in the mold of Trump than Marco Rubio or Jeb Bush.
It appears that Democratic leaders haven't yet lost control of their party. But the results of 2018 will be important. If largely party-picked candidates end up performing well and seize control of a chamber of Congress and a large number of state legislatures, some of the anti-elite tide may subside a bit. If not, though, Democratic activists might just draw a lesson from their Republican counterparts that they can win without their party's blessing.