Former White House counsel Don McGahn’s decision–on Donald Trump’s say-so–to defy an order to testify before a congressional committee could be the line which finally tips House Democrats to begin impeachment against Trump, according to prominent Democrats in Congress.

Leaving an empty seat before the committee Tuesday morning, McGahn defied a subpoena to testify before the House Judiciary Committee about the Mueller Report.

McGahn, who had been a key witness in special counsel Robert Mueller’s investigation, was told to skip the hearing by Trump.

McGahn’s defiance is just the latest among current and former Trump administration officials failing to heed the authority of Congress. And his absence from Tuesday’s hearing likely sets up another vote to hold a Trump official in contempt of Congress.

The committee has already voted to hold Attorney General William “Bill” Barr in contempt, also for defying a subpoena.

The White House is trying to shield McGahn from congressional testimony based on some claim of immunity, which is baseless since McGahn already offered his testimony to the Mueller team, Rep. David Cicilline (D-RI), a member of the judiciary panel, said in an interview ahead of Tuesday hearing.

“This claim of immunity, which is really a newly created claim, I think the White House recognizes executive privilege doesn’t apply since Mr. McGahn testified for 30 hours and of course, you can’t apply executive privilege after you disclosed the communications,” Cicilline said. “So now they’re making this really broad claim that essentially says the president and his team are immune from ever coming before Congress. That’s legally incorrect and certainly a court will ultimately have to decide that.

“But I think, let me be clear, if Don McGahn doesn’t testify, it is time to open an impeachment inquiry,” Cicilline added. “The president has engaged in an ongoing effort to impede our ability to find the truth, to collect evidence, to do our work, and this is preventing us really from ultimately finding the facts and doing our work in terms of oversight, and I think we now have seen a pattern where the White House is continuing an effort to really impede our search for the truth. No one is above the law, including the president of the United States, and we expect Mr. McGahn to come before the committee and testify.”

House Speaker Nancy Pelosi, thus far, had worked assiduously to tamp down any serious moves toward impeachment of Trump, largely due to the unknown political consequences of impeaching Trump in the House with the knowledge that the Republican Senate will never vote to remove him from office.

But Sen. Kirsten Gillibrand (D-NY) also believes continued stonewalling could create a path to impeachment. Gillibrand is one of nearly two dozen Democrats running for president.

“Yeah, I think what the Trump administration is doing is fundamentally blocking our constitutional right for oversight and accountability over his administration. And they’re doing it aggressively, certainly ignoring the contempt proceedings against Barr is a step, telling witnesses like McGahn that they are not supposed to testify,” said Gillibrand, also in an interview ahead of McGahn’s Tuesday no-show. “You know, I think it’s something that Speaker Pelosi — she has six committees of jurisdiction now that are trying to do oversight investigations and I think she will continue to push very hard to get testimony and documents. But if the president keeps stonewalling, it may actually force the House’s hands and they might decide to start a future proceedings because they need to get access to the truth and the American people have a right to know the facts.”