Democrat Beto O'Rourke, shown at an event last month, announced his presidential bid Thursday morning. (Carlo Allegri/Reuters)

Sen. Ted Cruz (R-Tex.) announced his candidacy for the Republican presidential nomination on March 23, 2015, 596 days before the 2016 general election. He was the first major-party contender to formally declare his candidacy that year. Former Florida governor Jeb Bush (R) had launched a PAC the previous December in hopes of boxing out other candidates by raising big money, but he didn’t actually announce until June. Donald Trump, the eventual nominee, didn’t announce until the day after Bush did.

We are currently 600 days from the general election in 2020. As of writing, there are at least a dozen serious Democratic candidates who’ve announced their presidential bids, depending on how you define “serious.” On Thursday morning, former Texas congressman Beto O’Rourke joined that group, announcing his candidacy after seemingly dragging his feet about the decision for a month. But if this were 2015, he’d have been the first one in the pool.

Perhaps the issue isn’t that 2020 is so early but that 2016 was late. Maybe, except the 2000, 2004 and 2012 fields also launched later than what we’re seeing so far in 2020. This year looks more like the entries into the 2008 election. By this far from the general election, eight Democrats and seven Republicans had already declared — and a Democrat, former Iowa governor Tom Vilsack, had already dropped out!

(Philip Bump/The Washington Post)

What do 2008 and 2020 have in common that might spur early presidential announcements? Both featured unpopular incumbent presidents, with George W. Bush’s approval rating at 33 percent two years from the election and Trump’s at 40 percent. But, then, Barack Obama’s approval two years before the 2016 election was also 40 percent.

Both the 2008 and 2020 contests also followed wave midterm elections in which one party (each time the Democrats) benefited from an energized base of support, motivated in part by opposition to the sitting president. That energy, it seems fair to assume, would be something that presidential candidates would want to capture. But, then, 2014 was also a strong election for the Republicans, picking up 13 seats in the House.

The 2012 election followed a much bigger wave for the Republicans — but Obama was sitting at a slightly better 45 percent approval two years prior. Perhaps, then, it’s the combination of an unpopular president and an energized electorate.

Another factor, clearly, is that the fields tend to move as a herd. If no one else is jumping in, there’s less urgency to do so.

(Philip Bump/The Washington Post)

In 2020, the Democratic National Committee introduced another motivation to get in early. Candidates can participate in primary debates if they’re at 1 percent in three separate polls — or if they’ve collected at least 65,000 individual donations from 20 states. The earlier you jump in, the easier that threshold is to meet.

There’s one other consideration: We don’t yet know how big the Democratic field will be. Most of the expected major candidates are in but not all. We could see another surge later in the year, which would change the question we posed at the outset to another one: Why did the 2020 campaign launch last so long?