Rep. Ben Ray Luján (N.M.), the No. 4 House Democrat, announced Monday that he's supporting an impeachment inquiry into President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE, making him the highest-ranking member of Speaker Nancy Pelosi Nancy PelosiDemocratic senator to party: 'A little message discipline wouldn't kill us' Overnight Health Care: New wave of COVID-19 cases builds in US | Florida to lift all coronavirus restrictions on restaurants, bars | Trump stirs questions with 0 drug coupon plan Overnight Defense: Appeals court revives House lawsuit against military funding for border wall | Dems push for limits on transferring military gear to police | Lawmakers ask for IG probe into Pentagon's use of COVID-19 funds MORE's (D-Calif.) leadership team to endorse the process formally.

Luján, who's leaving the House to run for a Senate seat in 2020, said he was swayed by warnings that Russian efforts to interfere in U.S. elections has continued heading into the 2020 cycle.

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Trump, he said, "has failed to act," making the launch of an impeachment inquiry a necessary step "to uncover the facts for the American people and hold this president accountable."

"The Trump presidency is creating grave national security concerns," Luján said in a statement. "Not only has he ignored the warnings that our Democracy is being targeted, but he has also actively encouraged Russian interference."

Earlier this month, the number of House Democrats officially backing impeachment ticked up to constitute a majority of the caucus. Luján's endorsement puts the figure at 123, and as assistant to the Speaker, he becomes the highest ranking Democrat to back the process — a distinction previously held by Rep. Katherine Clark Katherine Marlea ClarkDemocratic leaders: Supreme Court fight is about ObamaCare Rep. Robin Kelly enters race for Democratic caucus vice chair Clark rolls out endorsements in assistant Speaker race MORE (D-Mass.), the vice chairwoman of the caucus.

The slow drip of impeachment endorsements has put increasing pressure on Pelosi to get more aggressive as Democrats pursue a series of investigations into potential presidential wrongdoing, including conduct related to Russia's 2016 election meddling and the subsequent probe performed by former 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.

Mueller's report, released in April, found evidence that members of Trump's campaign team welcomed the foreign assistance in gathering "dirt" on Democratic nominee Hillary Clinton Hillary Diane Rodham ClintonBloomberg rolls out M ad buy to boost Biden in Florida Hillicon Valley: Productivity, fatigue, cybersecurity emerge as top concerns amid pandemic | Facebook critics launch alternative oversight board | Google to temporarily bar election ads after polls close Trump pledges to make Juneteenth a federal holiday, designate KKK a terrorist group in pitch to Black voters MORE, though it did not rise to levels of criminal conspiracy. Mueller also outlined 10 cases in which Trump may have obstructed the investigation.

With Pelosi's endorsement, Rep. Jerrold Nadler Jerrold (Jerry) Lewis NadlerDemocrats shoot down talk of expanding Supreme Court Schumer: 'Nothing is off the table' if GOP moves forward with Ginsburg replacement Top Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence MORE (D-N.Y.), chairman of the House Judiciary Committee, is leaning on impeachment as a basis for securing information from the administration, through the courts, to guide his ongoing investigations. But Democratic leaders have avoided votes to launch an official impeachment inquiry, citing the lack of support from both voters and Republicans on Capitol Hill.

Luján, who faces a contested Senate primary, said he was "alarmed" in reading Mueller's report, which revealed behavior "unacceptable ... from any president." He's worried, he said, that Trump isn't doing enough to ensure that the election meddling of 2016 doesn't resurface next year.

"President Trump's lack of action is jeopardizing our elections, national security, and Democracy," Luján said. "What is evident is that President Trump is abdicating his responsibility to defend our nation from Russian attacks and is putting his own personal and political interests ahead of the American people."