President Trump talks to chief strategist Stephen K. Bannon, right, and senior adviser Jared Kushner, center, during a swearing-in ceremony for senior staff at the White House in January. (Carlos Barria/Reuters)

President Trump has many conspiracy theories. And it turns out there's one of them that people in both parties buy into, bigly: the deep state.

A new Washington Post-ABC News poll shows about half of Americans (48 percent) believe in the concept of a deep state — i.e. “military, intelligence and government officials who try to secretly manipulate government policy” — and 35 percent dismiss it as a conspiracy theory.

The belief is very bipartisan. While 45 percent of Republicans believe it exists — perhaps believing it is undermining Trump even as we speak — 46 percent of Democrats also believe it exists — perhaps hoping it is undermining Trump even as we speak.

But it may not be a partisan thing, after all. Independents are slightly more likely to believe in the deep state, with 51 percent believing it's a real thing.

The biggest predictor for believing in the deep state is not partisanship but age. While 59 percent of those 18 to 29 years old believe in the deep state, 37 percent of those 65 and older do.

At the same time, many who believe in the deep state don't necessarily believe it's a big deal. Twenty-eight percent overall believe it's a “major problem” — including 29 percent of Democrats and 25 percent of Republicans. Another 14 percent say it exists but is a minor problem, and 6 percent say it's either not a problem or aren't sure.

Despite the apparently widespread belief in some kind of deep state, there aren't many politicians who are on record as agreeing. Top White House adviser Stephen K. Bannon is the most ardent believer in the White House. Rep. Mike Kelly (R-Pa.) was caught on tape last month alleging that former president Barack Obama was running a “shadow government” to undermine Trump, with his office later clarifying that he was merely sharing frustrations with fellow Republicans. Colorful former congressman Steve Stockman (R-Tex.) last month blamed his arrest for election law violations on the deep state.

As the New York Times explained last month, the concept of a deep state is more familiar in foreign governments beyond the first world:

The concept of a “deep state” — a shadowy network of agency or military officials who secretly conspire to influence government policy — is more often used to describe countries like Egypt, Turkey and Pakistan, where authoritarian elements band together to undercut democratically elected leaders. But inside the West Wing, Mr. Trump and his inner circle, particularly his chief strategist, Stephen K. Bannon, see the influence of such forces at work within the United States, essentially arguing that their own government is being undermined from within.

It is an extraordinary contention for a sitting president to make.

Except to the American people, it's apparently not so extraordinary.