The general understanding of the American electorate is that it’s about a 50-50 split, half Democrat and half Republican. There are third parties, of course, but we assume that about half the country belongs to one of the two parties.

That’s not the case. More Americans now identify as independents than as members of either political party, according to June data from Gallup. In fact, only about a quarter of the country identifies as Republican, and about 3 in 10 identify as Democrats.

That holds across multiple polls. The General Social Survey shows the long-term trend for membership in each party. Data from Pew Research Center has annual estimates since the early 1990s. Gallup has monthly estimates since 2004.

The trend is pretty clear: Membership in each party has declined over the past few years. The most recent figures in each poll are lower than the average since 2012, which itself is lower than the average since 2006. The figures are volatile, but the trend doesn’t favor the parties.

So if more Americans identify as independent than as members of either party, why aren’t third parties running the country? Moreover, how is it that the party with the lowest density in the electorate controls the White House, most state houses and Congress?

Two reasons. The first is that most independents still generally vote with one party or the other. The General Social Survey has a larger percentage of true independents (those who don’t lean toward one party or the other) than does Gallup, to the Republicans’ disadvantage. Gallup’s numbers, though, suggest that the split in the country is about 47-42, rather than 50-50, with the Democrats holding a slight lead when including independents who lean toward one party or the other.

So that’s one reason: A lot of independents vote for Republicans instead of third parties. (Why are they independents, then? Most are not hugely excited about the policies of the party they lean toward but also see the other party’s policies as harmful.)

The other reason is that Republicans have a built-in turnout advantage on Election Day. White voters often turn out more heavily than other racial and ethnic groups, as do older voters, wealthier voters and voters who own their own homes. All of those groups vote more heavily Republican than their counterparts: Whites more than nonwhites, older voters more than younger voters, wealthier voters more than poorer voters.

A recent PRRI poll found that 74 percent of those 65 and older were certain to vote in November, compared with 28 percent of those under 30. That obviously disadvantages Democrats.

As the graphs at the top of this article make clear, Democrats have long held a numeric advantage over Republicans, especially when including leaners. They just usually vote less heavily.

President Trump likes to talk about the silent majority that supports him. In reality, it’s a vocal minority.