Mr. Trump’s allies will try to dismiss the significance of Mr. Cohen’s cooperation, pointing out that he has now confessed to multiple crimes in two separate actions. But as any experienced legal practitioner knows, the government almost always relies on cooperators to make their case against their criminal cohorts. Often the best way of learning about, and proving, criminal activity is by working with those involved in that activity, unsavory as they may be. Plus, the documents filed Thursday quote extensively from emails that are almost certainly in the special counsel’s possession, which corroborates the belief that Mr. Cohen has come clean about his Russia dealings.

Mr. Cohen’s plea is also significant because it represents the first charge that Mr. Mueller is known to have pursued for false statements before Congress. Though other Trump associates, like George Papadopoulos, Rick Gates and Michael Flynn, also pleaded guilty for false statements, those were all made to investigators. A multitude of other Trump associates who gave testimony before Congress — such as Roger Stone and Donald Trump Jr. — now have additional reasons to worry about Mr. Mueller’s next moves.

Finally, Mr. Cohen’s cooperation continues to portend trouble for President Trump, the Trump Organization and the Trump political campaign, in conjunction with the payments Mr. Cohen arranged to women who claimed they had affairs with Mr. Trump. Mr. Cohen claims that these payments were made at Mr. Trump’s direction during the height of the 2016 campaign and that the Trump Organization reimbursed Mr. Cohen for related expenses in 2017. This also has profound implications for Mr. Trump because he is personally implicated in these deals, as are the entities that fueled his business and political success, and Mr. Cohen’s testimony could support criminal campaign finance charges against those involved.

The president and his lawyers may now be lamenting the game that they have been playing for nearly a year in delaying the president’s responses to Mr. Mueller’s questions. In so doing, they have given Mr. Mueller’s investigators time to flip multiple key associates. And the president now faces a new Congress that will be more likely to take action in the event that Mr. Trump tries to shut Mr. Mueller down, and diligently pursue the issues raised in any report released by Mr. Mueller and his team.

The potential risk to the president on three investigative fronts — obstruction, collusion and campaign finance — continues to grow. We know that the president has a tendency to hit back twice as hard when he is threatened. That puts the investigation at greater risk. Legislation to protect the special counsel was once again blocked by Republican leadership from a Senate vote on Wednesday. The coming omnibus spending legislation will provide a must-pass vehicle to try again to enact these protections, and we hope the supporters of Mr. Mueller, and of democracy and accountability, on both sides of the aisle will seize the moment. The course of justice may depend on it.

Barry Berke is co-chairman of the litigation department at Kramer Levin Naftalis and Frankel, where he is a partner specializing in white-collar criminal defense. Noah Bookbinder is executive director of Citizens for Responsibility and Ethics in Washington and a former federal corruption prosecutor. Norman Eisen is a senior fellow at the Brookings Institution, chairman of Citizens for Responsibility and Ethics in Washington, and author of “The Last Palace: Europe’s Turbulent Century in Five Lives and One Legendary House.”

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