Rep. David Cicilline David Nicola CicillineClark rolls out endorsements in assistant Speaker race Races heat up for House leadership posts The folly of Cicilline's 'Glass-Steagall for Tech' MORE (D-R.I.) is raising the stakes surrounding the Trump administration's stonewalling of House Democrats, calling for an impeachment inquiry against President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE if former White House counsel Don McGahn declines to testify before Congress on Tuesday.

"If Don McGahn does not testify tomorrow, it will be time to begin an impeachment inquiry of @realDonaldTrump," Cicilline tweeted.

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If he follows through on the threat, Cicilline, the head of the Democrats' messaging arm, would be the highest-ranking Democrat to endorse a formal effort to launch the process of ousting of the president — a campaign that Speaker Nancy Pelosi Nancy PelosiHoyer: House should vote on COVID-19 aid — with or without a bipartisan deal Ruth Bader Ginsburg lies in repose at Supreme Court McCarthy threatens motion to oust Pelosi if she moves forward with impeachment MORE (D-Calif.) and other party leaders have sought to suppress since Trump took office more than two years ago.

An overwhelming majority of Democrats support Pelosi's strategy, agreeing that impeachment is premature without more public support. They're pushing a series of aggressive investigations into the administration, including probes related to special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE's investigation into Russia's meddling in the 2016 elections.

Yet the drumbeat toward impeachment has grown louder since Mueller's report was released. And the administration's aggressive resistance to cooperating with the Democrats' investigations has caused more and more lawmakers to inch in that direction. The McGahn episode marks the latest case.

Democrats on the House Judiciary Committee had issued a subpoena to compel McGahn's appearance before the panel to discuss Mueller's investigation. But Trump has vowed the administration will be "fighting all the subpoenas," and the Justice Department made good on that promise on Monday, releasing a 15-page letter asserting that McGahn is not "legally required" to appear.

Cicilline, a Judiciary Committee member, has for months declined to endorse the push for impeaching Trump, and he voted against two impeachment-related measures on the House floor in late 2017 and early 2018. But the possibility of McGahn defying the Democrats' subpoena appears to be his tipping point.

Rep. Rashida Tlaib Rashida Harbi TlaibTrump attacks Omar for criticizing US: 'How did you do where you came from?' George Conway: 'Trump is like a practical joke that got out of hand' Pelosi endorses Kennedy in Massachusetts Senate primary challenge MORE (D-Mich.) had introduced a resolution in March requiring the Judiciary Committee to begin looking into impeachment. Before Mueller's report became public, only Rep. Al Green Alexander (Al) N. GreenThe Memo: Trump's race tactics fall flat Trump administration ending support for 7 Texas testing sites as coronavirus cases spike The Hill's Coronavirus Report: Miami mayor worries about suicide and domestic violence rise; Trump-governor debate intensifies MORE (D-Texas) had endorsed her bill. After the report was released, six others quickly signed on.

The resolution does not launch impeachment, per se, but asks the Judiciary Committee to investigate the president for potentially impeachable offenses. Supporters say it puts a finer point on the probes into potential administrative wrongdoing — and would lay the groundwork if the probes turn-up behavior that makes impeachment inevitable.

"It's seems clear to me that we need to be able to get to the bottom of this thing. The administration clearly is not going to help at all making it harder to be able to wade through this," said Rep. Earl Blumenauer Earl BlumenauerAhead of a coronavirus vaccine, Mexico's drug pricing to have far-reaching impacts on Americans Trump threatens to double down on Portland in other major cities Federal agents deployed to Portland did not have training in riot control: NYT MORE (D-Ore), who has endorsed Tlaib's bill.

"Our job, is to be able to explore the ramifications, lay the foundation for what might happen, and if you don't lay the foundation, it won't."