The lack of support for Sanders’ big ideas on health care, the environment and tax policy among Democratic centrists suggests major internal fights ahead.

“If he gets to be president of the United States ... there’s going to be some challenges to be able to get to a point where you can get to enough votes to get stuff passed,” said Sen. Jon Tester (D-Mont.), a centrist Democrat.

But if Sanders is elected president, he’s not going to simply lie down and submit to Congress — even if it’s controlled by Democrats.

Faiz Shakir, Sanders’ campaign manager and a former aide to both Reid and Pelosi, said party leaders should prepare for a President Sanders to “assume the responsibility for setting the agenda, creating the mandate for change, and pushing Congress to act.

“Sen. Sanders has respect for the burden of leadership that Schumer and Pelosi carry, but he also believes the party can and will need to act far more boldly for working-class issues in the years ahead,” Shakir said.

That tension is no different from what President Barack Obama faced from the likes of Sanders on the left and people like Sen. Joe Manchin (D-W.Va.) on the right.

“The system is designed to slow things down,” liberal Sen. Brian Schatz (D-Hawaii) said. “The system is designed to round out rough edges. And if we had a President Sanders, you would have a legislative branch that provided some of that friction.”

Pelosi has earned her own progressive bona fides. She often talks of marching in the streets decades ago, holding a protest sign in one hand and pushing a stroller in the other.

But Pelosi has only tepidly embraced some of the most radical progressive platforms — boasting about Obamacare when asked her opinion on Medicare for All and describing the Green New Deal as “the green dream or whatever they call it.” And she will not put legislation on the floor that would endanger Democrats’ House majority, no matter who is president.