The resistance to Sanders’ support for the "Green New Deal" and "Medicare for All" suggests major friction ahead in the campaign and on Capitol Hill. But there’s also little animosity and a feeling of mutual respect between Sanders and the leaders, according to interviews with a dozen Democrats on Capitol Hill this week.

“Sen. Schumer’s heart is with a lot of what Sen. Sanders is advocating. They may differ in their political calculus because they see the world through different prisms,” Sen. Richard Blumenthal (D-Conn.) said. “I think they’re very simpatico in terms of respect for each other.”

Sanders has disparate relationships with Schumer and Pelosi.

Sanders and Schumer both went to James Madison High School in Brooklyn and in Washington have regularly chowed down together with Sen. Sherrod Brown (D-Ohio) at Capitol Hill haunt Hunan Dynasty. They also both speak in loud, brash New York accents but otherwise couldn’t come off more differently in the world of Democratic politics. Schumer is an electoral animal and consummate dealmaker, while Sanders is about big ideas and a generally uncompromising style of liberal politics.

Sanders’ relationship with Pelosi is less involved. While Sanders served in the House alongside Pelosi for 16 years, he wasn’t the modern-day movement politician he is today. Pelosi and Sanders have a cordial working relationship and they, along with Schumer, teamed up to rally Democrats for a “day of action” to save Obamacare after the 2016 elections.

But the speaker and Sanders aren’t particularly close and their primary interactions during his House tenure involved committee assignments and other housekeeping issues, according to sources with knowledge of their relationship.

Accordingly, Pelosi didn’t need to develop as close a relationship with Sanders as Schumer, who became Senate Democratic leader just months after Sanders suspended his surprisingly strong presidential campaign against Hillary Clinton in 2016. But lawmakers close to Pelosi say if she can negotiate major funding and trade deals with President Donald Trump, working with a Democratic president — no matter how far left — is definitely achievable.

“We’ve seen how well she’s worked with folks. She’s even worked with this president, and she worked remarkably well with President Bush,” said Rep. Mike Thompson (D-Calif.), a Pelosi ally. “She’ll be fine with whomever the president is.”

But there are real hurdles ahead if Sanders becomes the nominee and president, not to mention fears among moderates that a Sanders nomination will hurt Democrats’ battle to keep the House and take the Senate.

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.

Democrats who have served with Sanders say his presidential agenda won’t be as dramatic as the policies he’s pushing on the campaign trail, particularly if Pelosi is speaker.

“I don’t think he’s, during the midst of a campaign where he’s stirred a whole movement, going to be talking about compromise,” said Connecticut Rep. John Larson, who served in Democratic leadership when Sanders was in the House. “But ultimately, he’s been here a while. He knows what he’s going to be up against.”

Sanders has drawn heat recently from former Vice President Joe Biden for encouraging a primary challenge to Obama, and he launched an infamous filibuster against Obama’s deal to extend Bush-era tax cuts. He’s also been relatively scarce around the Senate Democratic Caucus since his 2016 performance as he built his 2020 machine, according to senators.

But when the Democratic Party needs Sanders, he’s generally been there. And his signature accomplishment may be cutting a 2014 deal with the late Sen. John McCain (R-Ariz.) and former Rep. Jeff Miller (R-Fla.) on reforming the VA Department in the middle of a health care crisis, a development that belies Sanders’ reputation as a liberal hard-liner.

“He’s been a solid vote with the majority of the caucus most of the time,” said Sen. Dick Durbin, who has been Democrats’ chief vote counter during Sanders’ entire time in the Senate. “There have been a few [defections], you know. But it really is a limited number.”

If Sanders’ inside game in the Senate is lacking, it’s his outside game that earns the respect of his colleagues. With the Affordable Care Act on the ropes, Schumer and Sanders held a rally in Michigan that set the tone for Democrats from the center all the way to the left.

“Republicans, you're not going to destroy the Affordable Care Act,” said Sanders, who is now criticized for wanting to tear the law down with Medicare for All. “If you want to improve the ACA, then let’s work together.”

Two years later, Sanders and Schumer wrote a joint op-ed about stifling stock buybacks that enraged conservatives, then held a Facebook Live event to promote it. In a statement Schumer praised the insurgent-turned-frontrunner: “Bernie is a valuable voice in our caucus and an asset to our leadership team. He brings a very important perspective.”

Sarah Ferris contributed to this report.