Sen. Bernie Sanders (I-Vt.) is leaning into the term socialism in a way no other 2020 candidate is willing to do. He gave a speech Wednesday attempting to define his brand of socialism, invoking Harry S. Truman, Franklin D. Roosevelt, Lyndon B. Johnson and the Rev. Martin Luther King Jr. to make his case that Americans have always supported socialist-style programs without realizing it. (Social Security, Medicaid, Medicare, civil rights protections.)

“We must take on the unfinished business of the New Deal and carry it to completion,” he said.

It’s a risky move, not least because he has some convincing to do — among his 2020 Democratic field and the whole country. “Should the Democratic Party embrace socialism?” is at the heart of its debate about what kind of candidate can beat President Trump.

Let’s start with the whole country. Less than half of Americans say they’d vote for a socialist for president. More say they’d vote for an atheist or Muslim, according to a Gallup survey in May. That’s the same as 2015 when Sanders first launched his presidency, despite his efforts during that campaign to warm Americans up to socialism. A Monmouth University survey in April finds 57 percent of Americans think socialism is not compatible with American values.

Within the Democratic Party, it’s a different story. Sanders is successfully driving the conversation about what it should be. He’s forcing his primary opponents to take sides on an issue that polls show is popular with three-quarters of the Democratic base but unpopular with more than half the country. (That nearly 3 in 4 Democrats — 74 percent — are okay with socialism is a testament to Sanders, too. In 2015, Gallup had that number at 59 percent.)

Is this a train that can be stopped in its tracks? Some 2020 candidates argue Democrats are shooting themselves in the feet by embracing this term and its related policies, such as Medicare-for-all.

“If we’re not careful, we’re going to end up reelecting the worst president in American history,” said former Colorado governor John Hickenlooper. (He got booed by a gathering of Democrats in San Francisco for saying that.) It’s also notable that the most vocal proponents of dropping socialism from the campaign trail are at the bottom of the polls right now.

Former Colorado governor John Hickenlooper (D) was booed at the California Democratic Party's convention on June 1 after saying, "Socialism is not the answer." (Reuters)

Sanders argues that Trump has turned the political world upside down, and Democrats should, too. “If there was ever a moment when we needed a new vision to bring our people together in the fight for justice, decency and human dignity, this is that time,” he said Wednesday.

The rest of the candidates are somewhere in the middle, supporting big-government policies such as Medicare-for-all but eschewing the term “socialist” or “democratic socialist.”

Sanders has also done a remarkable job of changing the conversation in Washington, D.C. The House has held three hearings on his plan to have government run health insurance, Medicare-for-all (including one the same day as Sanders’s speech.) Four other senators who are his 2020 competitors have signed onto his Medicare-for-all legislation in the Senate.

Republicans are downright giddy about all this. What’s a scarier term to the American public than “socialist”? And with one of the leaders in the Democratic Party embracing it, their pitch to voters that Democrats want to radically change the country to something unrecognizable just got a lot easier.

“You add up things like packing the Supreme Court, getting rid of the electoral college, the Green New Deal and Medicare for none and you have a prescription of turning America into something it never has been and never should be,” Senate Majority Leader Mitch McConnell (R-Ky.) said recently. “So we intend to be on the offense in running our races."

“We’re going into the war with some socialists,” Trump told a fundraiser in April.

This could be the defining debate of the 2020 election. Trump is so happy with that. Sanders is fine with that. The rest of the candidates aren’t so sure. But given the way this debate is shaping up, they may have no choice but to pick a side.