Senator Bernie Sanders (Independent, from Vermont), introduced the latest version of his single-payer health care plan, with few details and only vague ideas about financing it. Jonathan Chait, in an excellent New York magazine column, argues that this means single-payer is “zero per cent closer” to passing, given that many Democrats have liked the idea for years but they’ve never been able to solve the policy and political problems involved in transitioning to it.

American political parties — the Democrats, in this case — are whatever their politicians, governing and campaign professionals, formal party officials and staff, activists and donors, and party-aligned interest groups and the partisan press want them to be. The most important way these party actors determine what the party will be is through nominations, with the most important of those being the presidential nomination. But conflict and cooperation over the party continues between elections, too, informed by how the last nomination contest played out.

The lifeblood of the party flows through extended networks; formal organisations such as the Democratic National Committee and its local counterparts are part of it, but there’s no overall top-down structure. People enter and leave the party all the time, and people and groups build up influence within the party in a variety of ways. Winning nominations is the best method of building influence, but bringing important resources such as money, constituencies, or expertise can win influence as well.

During the middle of the 20th century, parties were weaker and it was easier for individual politicians to make things happen without working inside the party system; powerful House chairs could advance ideas into law or block things from becoming law without needing permission from the party at large. The same was true of individual senators, who didn’t even need a committee perch to make something happen.

But parties grew much stronger within the political process over time, and Congress is now basically run by the parties; indeed, most things that happen in American politics, from elections through governing, flow through the parties. Major legislation, for example, is now in large part about getting into a party queue and building up support and formulating policy in preparation for when the party wins unified control. This period is likely to be relatively brief, so the first agenda items have the best shot of becoming law.

By any logic at all, another major overhaul of the health care system shouldn’t be on the Democratic agenda. It’s had its turn, rising to the very top priority the last two times Democrats had unified control of the federal government, which resulted in one failure in 1993 and the successful passage of the Affordable Care Act in 2010. Obamacare still (from the liberal point of view) needs additional legislative tinkering, but it’s been implemented, and it is extremely popular among Democrats and relatively popular overall. Meanwhile, other policy areas that were lower on the list in 2009 still need to be addressed — and more have emerged since. Advocates within the party demanding action on climate, income inequality, voting rights, immigration, infrastructure, campaign finance, and several other issues should have a strong case for pushing off anything major on health care.

But as it turns out, Sanders has placed single-payer squarely on the agenda, in large part by (somewhat retroactively) defining his surprising success in the 2016 nomination contest as mainly an endorsement of that policy. Parties, again, are self-defined in large part by nominations, and while Sanders fell well short of actually capturing his party’s standard, he did well enough to increase his influence.

So now the fight continues. Party activists, donors, interest groups, and governing professionals will push candidates who will be on the ballot in 2018, and are already pushing presidential candidates, to focus on some issues at the expense, inevitably, of others. Party politicians and campaign professionals will listen to them, and will also take their own readings about what voters in primary elections want and what voters in general elections will accept. The party-aligned media will weigh in too, influenced both by their personal preferences and, in many cases, by profit motives also (or, to put it another way, what their audiences respond to).

The role of presidential candidates is crucial, because they’re the ones, even more than members of Congress, who have strong incentives to put it all together and find bills which are strong enough to pass several tests:

— Rally partisans without risking the general election.

— Win enough congressional support to produce legislative victories.

— Succeed as laws well enough that they can run on them in the reelection campaign.

Since presidential candidates (to some extent) and presidents (to a larger extent) can make their own contributions to the fight over policy choices and priorities, all party actors fight hard to make sure the nominee is at least acceptable to them.

We can see that even in the unusual case of Donald Trump, who seized the Republican nomination over the objections of most party actors and then run a presidency which is far less connected to the party by personnel than any other recent president. Trump has nevertheless been constrained to a large extent by the Republican agenda — unorthodox policy positions such as infrastructure and trade have faded, while his priorities have been the Republican priorities of repealing Obamacare, cutting taxes, and securing Republican influence on the courts. At the same time, he’s certainly in a position to change Republican policy preferences and priorities, and if he were better at his job, he would likely achieve some of that. Or to put it another way, because parties are so strong, for Trump to be an effective president he’ll have to learn to be an effective Republican.

Sanders’s event, then, is best seen as a part of that ever-continuing fight over what the party will really stand for.

— Bloomberg

Jonathan Bernstein is a Bloomberg View columnist. He taught Political Science at the University of Texas at San Antonio and DePauw University and wrote A Plain Blog About Politics.