Sen. Bernie Sanders (I-Vt.) smiles before he speaks during a primary-night watch party at Concord High School, Tuesday, in Concord, N.H. (John Minchillo/Associated Press)

Lawrence Lessig is a professor at Harvard Law School and author of “Republic, Lost: Version 2.0.” In November, he ended his campaign for the Democratic nomination for president.

Bernie Sanders has made great strides casting doubt on the credibility of Hillary Clinton as an agent of change. How can you take on Wall Street if you take quarter-million-dollar speaking fees from its leading banks? How can you be a credible reformer if you have been so dependent on money from the status quo?

But Sanders has his own credibility problem. It’s called Congress. The Vermont senator’s agenda is a “fiction,” the Post editorial board declared, because there is zero chance it could get through the legislature, and not just because there are more Republicans than Democrats on Capitol Hill. Even when President Obama had a super-majority of Democrats in Congress, he couldn’t get climate change legislation passed or a public option included in Obamacare. The threat of the powerful energy and health-care industries pouring millions of dollars into campaigns against Democrats was enough to get the leader of the last great “revolution” in U.S. politics to stand down. Until we change the way that money matters on Capitol Hill, the more sober-minded — they call themselves “realists” — will just roll their eyes at the fantastical promises of America’s most authentic politician.

Sanders should have an easy response to this reality-based skepticism: His first move as president, he should insist, would be to get Congress to change the system that so systematically blocks real reform.

And, indeed, that was precisely what Sanders seemed to say in the fifth Democratic debate. When asked by Chuck Todd what his priority as president would be, Sanders was quite clear: “You’re not going to accomplish what has to be done for working families and the middle class unless there is campaign finance reform. So long as big-money interests control the United States Congress, it is going to be very hard to do what has to be done for working families.”

No truer words have been uttered by a candidate for president — at least since Obama said essentially the same eight years ago: “If we’re not willing to take up that fight [to change the way Washington works], then real change — change that will make a lasting difference in the lives of ordinary Americans — will keep getting blocked by the defenders of the status quo.” The kind of systemic, fundamental change that Sanders now, and Obama then, are talking about will require a fundamental change in the economy of influence in Congress. It was enormous progress when Sanders made that clear.

But on the same day that Sanders declared that campaign finance reform must come first, his campaign released a statement completely negating the significance of that promise. Responding to a crowdsourced question on change.org’s new politics platform, Sanders again promised to “commit to making reforms that change the way campaigns are funded a primary objective” of his administration. But when he listed what he would actually do in his “first 100 days in office,” his list included three minor changes related to transparency in political spending. As to the only change that could make his platform credible — revamping the way congressional elections are funded by adopting a system of small-dollar, citizen-funded campaigns, such as the one Rep. John Sarbanes (D-Md.) has proposed — Sanders indicated this was something to “move toward” “over the long term.”

“Over the long term”? What exactly does Sanders expect to accomplish in the short term, before this change is enacted? As Sen. Elizabeth Warren (D-Mass.) said in a speech on the floor of the Senate on the anniversary of the Supreme Court’s Citizens United decision, this is not a fight for the long term. This is the fight to be having right now. The only way real change will happen in the United States is if Congress is set free from its corrupting dependence on interested money. Yet, bizarrely, Sanders doesn’t commit to promoting this essential change in Congress as a priority of his administration.

Like Obama before him, Sanders has excited a progressive base with powerful ideas about how to improve the United States. Like Obama before him, he has attacked the money from special interests that so powerfully defends the status quo. But like Obama before him, Sanders has failed to make central the one change that could make his revolution credible: changing the way congressional campaigns are funded. Progressives looking for a revolution should heed Warren’s words: This is not a change for “the long term.” This is a fight to be having now.