With Democratic primary debates having begun on Wednesday and continuing on Thursday, featuring 20 candidates, the party is facing its largest field of candidates in modern history. But part of the reason this moment stands out is that this kind of crowded and competitive field is a relatively recent phenomenon.

The potential for having so many candidates in contention is the result of party reforms in the early 1970s. Those changes were made after Americans took to the streets when Hubert Humphrey became the Democratic nominee at the 1968 Democratic National Convention in Chicago without having campaigned in any primary races.

“Primaries in the old system were few and far between,” says Elaine Kamarck, Senior Fellow at the Brookings Institution and author of Primary Politics: Everything You Need to Know about How America Nominates Its Presidential Candidates. When primary elections did happen, they were “beauty contests” used by the party to test electability, but were separate from the selection of a state’s delegates to the party’s nominating convention. “The old nominating system was controlled ultimately by elected officials and party officers of the Democratic party,” sitting down with the governor in a smoke-filled room. That changed for Democrats between 1972 and 1976, as primaries became the dominant way of determining delegates and thus nominating the candidates. (The Republican party also made some reforms to reduce the power of party bosses, but overall, the party didn’t undertake the kind of reforms that Democrats did until Democratic-controlled state legislatures made more open primary systems the law, according to Kamarck.)

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And the crowded field isn’t the only parallel between 1976 and today, Zelizer says. The then-unprecedented number of candidates came during unprecedented times. Then, Watergate shook up political norms; now, President Trump has changed the conversation in his own way. “Among Democrats there’s a sense of crisis,” he says of the 2020 race. “I think that encourages lots of people to participate.”

The 1976 primary would have a ripple effect on future Democratic primaries.

After the unicorn candidate Carter lost re-election in 1980 to Ronald Reagan four years later, there was a sense that establishment types were the safer bet. Superdelegates — selected to attend the convention and vote, but free to support anyone they want and change their minds at any time — were added to the nomination mix as one way to give party leaders and elected officials more of a say in choosing the nominee. (In 2018, the Democratic Party scaled back their powers in response to concerns that these insiders skewed the votes for the 2016 nominee Hillary Clinton.)

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As Kamarck sees it, “Most of the races on both the Democratic and Republican side [of history] have been composed of people who had a plausible claims to be President. If you think of nominees between 1972 and 2012, most of them are people who might have won under the old system,” she says. “You used to have to have enough of a national reach to raise a fair amount of money to move around country and meet voters. These days, running a campaign online costs virtually nothing.”