Bernie Sanders has risen to the top of the Democrats’ 2020 pack on the appeal of his far-left idealism and promises of a “revolution” — but he’ll have a hard time turning revolution into reality if he gets the chance.

The big picture: Even with the expanding power of the presidency, Sanders would need Congress to approve the most ambitious ideas he’s known for — and that’s unlikely to happen even under the strongest elections scenarios for House and Senate Democrats in November.

Medicare for All, the Green New Deal, free college and other Sanders proposals that excite the Democratic base would likely hit a logjam in the Senate — even with his plans to make expansive use of the power of budget rules to push his ideas through with a bare majority.

Internal Democratic divisions would threaten his ability to get even a bare Senate majority for them, let alone ram them through the expected wall of Republican opposition, according to moderate Democratic senators, Democratic aides and Axios experts.

"He’s not gonna get that," said Sen. Joe Manchin of West Virginia, one of the moderate Democrats whose support Sanders would need. "Bernie has a little different, unusual approach to fixing [the country’s problems], more so than what I could be acceptable with."

And that assumes he could get them through the House without major substantive changes — which is no sure thing either.

Meanwhile, some establishment Democrats worry that nominating Sanders would sink their chances of recapturing the Senate — and shrink or cost their majority in the House.

Yes, but: Don’t write off Sanders' chances to make some major changes in direction on his own. He could likely reverse course on many of President Trump’s immigration policies through his executive authority, by stopping the construction of the border wall and not prosecuting border crossings.

And Sanders' advisers say he could make major changes on drug prices and climate policies through regulations and executive orders — like declaring a climate emergency, which could give him far more authority to spend on climate measures without congressional approval.

He may also use executive orders to advance priorities like the $15 minimum wage, by applying it to government contractors.

But those aren’t the major themes that have defined his campaign, which he describes in much more ambitious terms — “health care as a human right,” college for all, jobs for all, and “an agenda that works for the working people of this country, not for the billionaire class.”

Even on policies that could have broader appeal — such as wiping out medical debt and expanding Social Security — "I can't imagine any of those getting the sufficient votes to even get a majority, much less getting 60 votes," said Sen. Doug Jones of Alabama.

The other side: There's one potentially game-changing variable, as Trump's takeover of the Republican Party has shown us: A big Sanders win could change the political fault lines, if moderate Democrats become less likely to vote against him for fear of alienating the Democratic base.

Sanders' advisers say any Democratic president would face similar obstacles, even with a more moderate agenda. But they argue that Sanders can break through them by mobilizing pressure from his supporters and using the budget rules to give him enough leverage to end the cycle of opposition.

"He has a theory of how to do that — not just building a movement, but using it to pass legislation," said Josh Orton, national policy director for the Sanders campaign.

If that sounds like a stretch, Orton said, "it certainly is more likely than just hoping Mitch McConnell suddenly comes to his senses" and allows more moderate Democratic policies to pass.

The details: The Sanders team knows the Senate will be a problem, thanks to (what's left of) the filibuster. They’re planning to get around that by using budget reconciliation rules to pass most of his major plans — channeling progressive Democrats’ frustrations over the Senate’s track record of killing big plans.

Those rules can allow proposals to be passed with just a majority vote, but only under strict conditions. Anything that doesn’t have an impact on spending or revenues can be knocked out, and some problems can’t be fixed, as Democrats found when they used it to finish passing the Affordable Care Act in 2010.

Sanders plans to get around that problem by directing his vice president to rule that Medicare for All, the Green New Deal and other big proposals can be passed that way — overruling any procedural challenges that might come up.

The catch: Even under a narrow Democratic majority in the Senate — which is the best-case scenario right now given the direction of this year’s Senate races — Sanders would need to win every Democrat to get that majority support.

That would be an extreme challenge given that virtually all of his most ambitious proposal have divided Democrats on the campaign trail and would be sure to divide them in the Senate.

Even if he managed to do that, there would be fallout within the Senate from repeated use of the vice president’s powers to overrule any procedural objections — and not just among the senators.

The vice president would have to repeatedly overrule the Senate parliamentarian, a non-political officer whose job is to interpret the rules. If that happens again and again, the parliamentarian could just step aside and stop offering advice on the next procedural challenges — basically telling the vice president: You’re on your own.

“If they ignore my advice, it becomes difficult to continue giving advice,” former Senate parliamentarian Alan Frumin told Axios, gaming out how that scenario might work. “You go that way, you go by yourself.”

The bottom line: Sanders’ supporters aren’t wired to obsess about arcane Senate rules, a parliamentarian’s feelings or other Washington insider issues. But they will care if they vote Sanders into the White House expecting a revolution and find themselves wondering what's taking so long.

Go deeper:

Reality check on Bernie Sanders' biggest ideas

The steep odds against Elizabeth Warren's big 2020 ideas

Reality check on Elizabeth Warren's biggest ideas