One reason why Democrats have been so unified is that there are structural reasons to expect a majority caucus to be more cohesive than a minority one. For one thing, minority-party members’ “votes don’t make or break legislation a lot of the time,” Gregory Koger, a professor of political science at the University of Miami, told FiveThirtyEight, so “there is a little more leeway for them to break with their party.” For another, majority-party members (especially when the president is also of that party) have a clear electoral incentive to get things done. “All Democrats — regardless of whether they’re moderate or progressive — really need the Biden administration to succeed,” said Ruth Bloch Rubin, a political science professor at the University of Chicago. But on the other hand, the minority party has “competing incentives: They want the president to look ineffective but also want to bring things home to their district.”

Perhaps most importantly, the majority party also has an advantage in that it sets the congressional agenda, and congressional leaders don’t typically bring bills to the floor unless they are sure they are going to pass. As a result, only bills with broad support within the caucus get voted on, making the majority party look more cohesive than it would if the minority party was calling the shots. 

In some ways, however, this is a key shortcoming of our Biden Scores: They don’t measure the votes not taken or what goes on behind the scenes. “In historical cases, a lot of really important negotiation happens before legislation hits the floor,” said Bloch Rubin. “Not to undercut the value of looking at final votes … but a lot of the time that’s not where the most important action is.” In fact, a real-life example of that is unfolding before our eyes right now in the Senate, where it’s likely that the opposition of moderate Sen. Joe Manchin will force Democrats to lop off a trillion dollars or two from the reconciliation bill. (Manchin, though known as one of the biggest internal thorns in Democrats’ sides, has a 100 percent Biden Score.) A similar dance occurred with Democrats’ voting-rights bill earlier this year: The For the People Act was too far-reaching for Manchin’s tastes, so it was pared down into the less ambitious Freedom to Vote Act, which Manchin helped craft and is now likely to support.

So it makes good sense that Democrats in the 117th Congress are more united than Republicans are in the 117th or Democrats were in the 115th. But it doesn’t explain why they are even more united than the Republican majority was in 2017-18. During that 115th Congress, the middle 90 percent of Republicans (so again disregarding the outliers in the top and bottom 5 percent) had Trump Scores between 81 and 99 percent. That 18-point range is not nearly as narrow as the 3-point range that separates the middle 90 percent of the current Democratic caucus. Put another way, Republicans were a bit more cohesive when they had the majority than they are now — but Democrats are a lot more cohesive now than when they were in the minority.

Why have Democrats been so successful at keeping their caucus in line? Koger sees two reasons: the fact that Democrats’ majority is so narrow (there are only eight more Democrats in the House than Republicans) and Republican opposition to the Democratic agenda. “If Pelosi could count on 20, 30 or 40 percent of Republicans to vote for a bill, there would be less pressure on Democrats to unite,” Koger said. But now, “when the majority party wants to do something, that typically involves corralling all its members.” 

In other words, a larger majority means leadership has a wider margin of error; they can afford to not whip votes as aggressively or to allow members to vote their conscience (or cast a vote that might be more defensible to their constituents). But when a House majority is this narrow, there is more pressure to toe the party line. “It’s a lot of pressure to go out on the floor and sink a vote,” Matt Glassman, a former congressional staffer and now a senior fellow at Georgetown University’s Government Affairs Institute, told FiveThirtyEight. “Everyone knows they can do it, but everyone’s terrified of doing it.” 

As such, both Bloch Rubin and Glassman expressed skepticism that progressives would ultimately scuttle the infrastructure bill. “They all want this infrastructure bill to pass,” said Bloch Rubin, and Glassman likened the negotiations to a performance where “everyone is trying to get to yes” but publicly threatens to vote no in order to “move policies incrementally closer to where they want them to be.” 

In this, progressive Democrats differ from their far-right counterparts in the House Freedom Caucus, who are more doctrinaire and willing to torpedo their party’s agenda — for instance, voting down a 2018 farm bill because then-House Speaker Paul Ryan did not first hold a vote on a controversial immigration bill. “The progressive caucus has never really wanted to take the next step and fight stuff on the floor,” Glassman said. “[They] work within the system.” This fundamental difference between the parties’ extreme flanks is another big reason why this Democratic majority is more cohesive than the last Republican one.

Pelosi, who has a reputation as a master legislative tactician, deserves some credit as well. “I would hardly say that structural factors alone are doing the work,” said Bloch Rubin. “She knows how to play her cards well.” In fact, Glassman told us that Pelosi’s unique strength isn’t in whipping votes; instead, both he and Bloch Rubin pointed to her ability to manage the factions within the Democratic coalition. Her job is essentially negotiator-in-chief, “making sure all members of the coalition are OK with the outcome,” Glassman said. “And I think Pelosi is very skilled at making sure everyone gets just enough of what they want.”

Ironically, though, an open negotiation process like the one Democrats are currently in can leave outside observers with the impression that a party is divided even if the legislation being debated ultimately succeeds. “It’s very hard to tell the difference between a caucus that’s in disarray and one that’s in the late stages of bargaining with each other,” Glassman said. “The visible evidence is the same.” Media coverage of the negotiations usually doesn’t help matters, either; according to research by political scientist Mary Layton Atkinson, the press covers controversial legislation far more often than it does bipartisan legislation, and that coverage generally focuses on the conflict and drama of the negotiations over the substance of the bill. To Bloch Rubin, though, this type of coverage misses the broader point that Pelosi has proven adept at steering Democrats’ squabbling factions toward an outcome that actually benefits her party in the end.

Glassman identified one other reason why Democrats appreciate and appear so unified under her: “She protects them from votes they don’t want to take on the floor.” In other words, she is good at agenda-setting and not holding votes until negotiations are complete. We saw an example of this just days ago: The infrastructure vote was actually originally scheduled for Monday, but Pelosi postponed it until Thursday in order to buy more time for negotiations. “I’m never bringing to the floor a bill that doesn’t have the votes,” she explained to ABC News.

All this is not to say you should ignore the very real policy differences between Democrats’ moderate and progressive flanks. (For one thing, they’ll continue to be an important fissure in Democratic primaries in 2022 and 2024.) But those divisions popping up in Congress does not necessarily make Democrats ineffective at governing. Negotiations, by definition, highlight disagreements, but the final proof will be in whether Democrats pass the infrastructure bill on Thursday (and, on some later date, the reconciliation bill). 

In other words, it’s possible for a party to have divisions but not be divided — and a strong congressional leader like Pelosi can make that happen.