This post is part of Polyarchy , an independent blog produced by the political reform program at New America , a Washington think tank devoted to developing new ideas and new voices.

Now there are 21. With former Vice President Joe Biden and Rep. Seth Moulton joining the Democratic presidential race last week, and Sen. Michael Bennet announcing on Thursday, the number of candidates seems to have set a record. “Can you explain why the dam has broken, and everyone is a candidate in the Dem primaries?” a Twitter follower asked me last week.

My first response was a banal one, which is that the number of candidates really isn’t so unusual for an election like this one. Political opportunities like 2020 don’t pop up very often. There’s no Democratic incumbent seeking reelection, no obvious next-in-line Democrat (like Al Gore in 2000 or Hillary Clinton in 2016), and there’s also a solid, though not certain, chance of Democratic victory. In the recent past, whenever those three conditions have aligned, quite a few candidates have been lured into the race.

At this point in 1987, for example, nearing the end of Ronald Reagan’s second term, there were 13 candidates, including many whose names would occupy the highest difficulty tiers of any political trivia contest. Who remembers former Rep. Doug Applegate of Ohio? Probably not even the 25,000 people who voted for him in 1988.

Because it’s a big chapter in the Biden saga, many know he was a candidate that cycle before dropping out in September 1987, but few remember a remarkably qualified candidate who stayed in a few days longer: Rep. Patricia Schroeder of Colorado, an eight-term Congress member who was an expert on defense modernization as well as an early advocate for family leave and child care. Similar years with open Democratic fields include 1976 (15 announced candidates), 1992 (nine candidates), and 2008 (eight candidates).

But none of those fields reached 21 candidates. There may be a few other reasons that this cycle is even more attractive to candidates than similar situations in the past. Six candidates are sitting senators. That’s more than in any cycle since 1976, when five of the most distinguished and mostly admirable senators of the 20th century received just 15 percent of the Democratic primary vote, combined.

The big field of senators this time probably has something to do with the nature of the current Senate. While it’s an old (and obviously sexist) joke that every senator sees a president when he shaves in the morning, many senators consider running but decide they can have plenty of impact in the institution, either by rising to chair a committee or by carving out an entrepreneurial role around a particular issue or perhaps investigations. But in Mitch McConnell’s Senate, the scope of action for any individual senator is profoundly limited.

They usually can’t even offer amendments on the floor! Low-seniority senators such as Cory Booker and Kamala Harris have very little to lose by running for president, even if it means a year and a half of missed votes. The same is probably true of the House of Representatives. Of the four sitting House members in the race for the White House, two, Tim Ryan and Seth Moulton, have at times aspired to move into leadership. But Democratic leadership in the House has been frozen in place since the triumvirate of Nancy Pelosi, Steny Hoyer, and Jim Clyburn cut a deal 15 years ago.

For these members, life in the minority during most or all of their careers has been a miserable exercise in total irrelevance, but life in the majority this year, where first-year members such as Alexandria Ocasio-Cortez are drawing all the attention, might not be much better for mid-career politicians such as these. Like their Senate peers, why not roll the dice on the presidency?

There’s also money — there’s just more of it than ever, more dollars and more donors. In the recent past, the biggest influence of money on presidential elections involved its scarcity. The finance chair of former Rep. Dick Gephardt’s unsuccessful 1988 campaign told a researcher later, “Candidates don’t lose elections. They run out of money and can’t get their planes off the ground.”

That was then. Beginning roughly with the Howard Dean campaign in 2004, the number of people willing to give money to candidates has skyrocketed. Because of the requirements for participation in Democratic debates, candidates are now focusing on the total number of donors, with four already surpassing 100,000 contributors and Bernie Sanders over the half-million mark more than 18 months in advance of the election.

But the number of medium donors, those giving more than $200 but within the hard-money maximum (currently $2,900), and reported by name to the Federal Election Commission, has also grown. In 2000, only about 778,000 people made reportable donations to any federal campaign. By 2008, with Barack Obama taking Dean’s achievement much further, that number had almost doubled, to 1,337,000, and in 2016, 1,672,000 people donated more than $200.

Even as Democratic candidates have challenged themselves and each other by renouncing various forms of money, whether from PACs, organized fundraising events, lobbyists, or people employed in the oil and gas industries, they are operating under conditions of abundance rather than scarcity.

Just as a Super PAC and a single donor in 2012 kept Newt Gingrich’s candidacy alive long past the time when voters had lost interest (the opposite of Gephardt’s problem in 1988), the sheer number of small and medium donors available to Democratic candidates makes it much easier to get in the race and stay in.

The structure of campaign expenses, in which paid broadcasting ads are less central and online and on-the-ground organizing more valuable, might also affect the way political money works, not necessarily making campaigns cheaper but making it easier to start with less and lowering the barrier to entry. A broad base of smaller donors has another advantage: Candidates can go back to them for more, until they reach the limit of what each donor is able to give.

Finally, let’s not be totally cynical about politicians and their motives. They’re not always automatons fueled by pure ambition. Genuine outrage about the current direction of the country, along with a real hope that we can do better and escape the dreary constraints of even Democratic politics in the Clinton and Obama eras, surely has mobilized or inspired some candidates to enter the fight. That sense of obligation and inspiration was absent in earlier election cycles I can remember, such as 1988 or 1992.

Inevitably, quite a few promising candidates who would make good presidents, perhaps a dozen, will fall away long before the primaries and caucuses begin. Pundits will draw conclusions, such as that their message or policy ideas were unpopular with Democratic voters or that the party “doesn’t have room” for a moderate, a woman, or whatever might characterize them. But more likely, it will be a small tactical stumble — they hired staff that wasn’t ready for a presidential campaign or they picked the wrong states to focus resources on.

When former Sen. Birch Bayh died in March, I read the obituaries eagerly, in part curious how such an accomplished senator (he passed two constitutional amendments and almost passed a third, the Equal Rights Amendment) disappeared so quickly in the 1976 campaign. The answer was boring, unfortunately: He hadn’t figured out how to work within the new fundraising rules of the 1974 Federal Election Campaign Act.

It’s good that Democrats have so many choices, and it’s good that the barriers to entry in politics at the highest level seem to be somewhat lower than in the past. Unfortunately, our first-past-the-post electoral system makes it hard to sustain all those choices through the primaries, and means that we will lose promising candidates while the final few will be somewhat arbitrary.

If having 20-plus candidates becomes the new norm, the party should look at reforms such as ranked-choice voting that will give voters a chance to express their support for more than just one candidate at a time.

Update 5/2: Updated to reflect that Michael Bennet has now also announced his campaign.