House Democrats of all stripes say the window for launching impeachment proceedings against President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE is quickly closing.

Heading into the long August recess, Democratic leaders resisted efforts to initiate a formal impeachment inquiry, frustrating liberals who say 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 report on presidential obstruction — accentuated by the former special counsel's testimony before Congress on Wednesday — provides ample reason to begin the process.

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Those progressive lawmakers are hoping Mueller's remarks will sink in with voters over the six-week summer break, nudging more House Democrats to endorse impeachment when Congress returns to Washington in September — and force leadership to take it up.

To wait much longer, they warn, will be too late.

"If we don't take action come Sept. 1, then we should just shut it down because we're not going to be able to do anything at all," Rep. Jackie Speier Karen (Jackie) Lorraine Jacqueline SpeierOvernight Defense: House to vote on military justice bill spurred by Vanessa Guillén death | Biden courts veterans after Trump's military controversies House to vote on 'I Am Vanessa Guillén' bill Overnight Defense: Trump's battle with Pentagon poses risks in November | Lawmakers launch Fort Hood probe | Military members can't opt out of tax deferral MORE (D-Calif.), a member of the House Intelligence Committee, told CNN. "I feel strongly that we should, but I think we're running out of time."

Other Democrats see a longer timeline for Congress to move forward on impeachment — but not much longer. With the 2020 presidential race ramping up, they say, the utility of impeachment has an expiration date.

"I think it's got to be this year. You can't start it after the Iowa caucuses," said Rep. Ro Khanna Rohit (Ro) KhannaThe Hill Interview: Jerry Brown on climate disasters, COVID-19 and Biden's 'Rooseveltian moment' Congress needs to prioritize government digital service delivery DeJoy defends Postal Service changes at combative House hearing MORE (D-Calif.), who has backed leadership's more cautious, investigative approach to executive oversight. "I think we've got to make a decision probably before Thanksgiving."

Behind Speaker Nancy Pelosi Nancy PelosiPelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' On The Money: Anxious Democrats push for vote on COVID-19 aid | Pelosi, Mnuchin ready to restart talks | Weekly jobless claims increase | Senate treads close to shutdown deadline Trump signs largely symbolic pre-existing conditions order amid lawsuit MORE (D-Calif.), Democratic leaders have rejected the move toward impeachment, citing the need to build a stronger case of presidential wrongdoing before taking such a divisive step. They've pushed for a series of aggressive committee investigations, combined with a string of legal actions, designed to yield more evidence of Trump's misconduct — and perhaps shift public sentiment in favor of impeaching him.

Pelosi is also fighting to protect the vulnerable swing-district Democrats facing tough reelection next year — a group concerned that impeachment would harm their chances at another term. With that in mind, some liberals have accused Pelosi of attempting to bury impeachment for political reasons ahead of 2020 — a charge she rejected on Friday.

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"I'm not trying to run out the clock," she told reporters at the Capitol. "We will proceed when we have what we need to proceed. Not one day sooner."

Many critics of the president were hoping Mueller's back-to-back testimony before the House Judiciary and Intelligence committees on Wednesday would provide the kind of bombshell revelation to accelerate the impeachment effort.

“It didn't happen," acknowledged 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), a vocal impeachment supporter.

Still, despite Mueller's halting delivery, the substance of his testimony offered Democrats plenty of fodder for their critiques. Amplifying the most damning findings of his report, Mueller told lawmakers that Moscow meddled in the 2016 elections for the purpose of helping Trump; that members of Trump's inner circle enthusiastically accepted the foreign support; and that Trump lied about that support and tried to shut down the investigation in ways that might constitute criminal obstruction.

Since those hearings, at least seven Democrats have signed on to the impeachment effort, including 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 (Mass.), the sixth-ranking Democrat in the House. Those endorsements have pushed the number of Democratic impeachment supporters to 99, according to a tally kept by The Hill.

Several Democrats said that figure will only rise in the coming days, as lawmakers gauge the post-Mueller temperature of voters in their districts.

"There are at least 20 to 30 [lawmakers] that are going to be doing a lot of ... deep contemplation, as am I, in the next few days," Rep. Denny Heck Dennis (Denny) Lynn HeckExclusive: Guccifer 2.0 hacked memos expand on Pennsylvania House races Heck enjoys second political wind Incoming lawmaker feeling a bit overwhelmed MORE (D-Wash.) told CNN on Friday. "This was a pretty high impact week."

Heck cautioned that, for Democrats hoping to hold Trump into account, impeachment has a shelf-life.

"There's certainly some point in time that ... it wouldn't probably make much sense to proceed with it," Heck said, without specifying a date.

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 Judiciary Committee, has reportedly pressed for impeachment proceedings on his panel only to be rebuffed by Pelosi. On Friday, the pair seemed to reach some compromise, as Nadler laid out a strategy of explicitly citing the possible impeachment of Trump as a basis for a legal request to secure grand jury information related to Mueller's investigation.

That's empowered liberal Democrats to talk tougher about impeachment without forcing any votes that could hurt moderates.

“I would say we are in an impeachment investigation,” Rep. Jamie Raskin Jamin (Jamie) Ben RaskinOVERNIGHT ENERGY: House passes sweeping clean energy bill | Pebble Mine CEO resigns over secretly recorded comments about government officials | Corporations roll out climate goals amid growing pressure to deliver On The Money: House panel pulls Powell into partisan battles | New York considers hiking taxes on the rich | Treasury: Trump's payroll tax deferral won't hurt Social Security House panel pulls Powell into partisan battles over pandemic MORE (D-Md.), a Judiciary member and former constitutional law professor, said Friday.

Still, that strategy likely won't satisfy the most ardent impeachment supporters, who have grown frustrated with the near-blanket refusal of the administration to cooperate in the House investigations and want party leaders to launch a formal impeachment inquiry.

Pelosi, cognizant of those concerns, has "green-lighted" Democrats to support impeachment in recent days, in the description of Rep. Jared Huffman Jared William HuffmanOVERNIGHT ENERGY: House passes sweeping clean energy bill | Pebble Mine CEO resigns over secretly recorded comments about government officials | Corporations roll out climate goals amid growing pressure to deliver OVERNIGHT ENERGY: House Democrats tee up vote on climate-focused energy bill next week | EPA reappoints controversial leader to air quality advisory committee | Coronavirus creates delay in Pentagon research for alternative to 'forever chemicals' COVID-19 complicates California's record-setting wildfire season MORE (D-Calif.), an impeachment supporter. And she's also vowing that the committee investigations — and the subsequent decision of whether to impeach Trump based on the findings — won't run forever.

"The decision will be made in a timely fashion," she said Friday. "This isn't endless."

Huffman predicted dozens of Democrats are ready to jump on board the impeachment bandwagon in the coming days and weeks. When that happens, he said, leaders will face intense new pressure to act on the appeal. And he's predicting they will.

"I think September is crunch time," Huffman said.