Democrats considered potential presidential candidates say if President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE fires 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 it would be grounds for impeachment.

The would-be candidates, who have been careful in their comments about the politically thorny issue, have now begun to qualify on what grounds they would push for impeachment.

And they are signaling that terminating Mueller is a red line.

At a town hall in Sacramento, Calif., last week, Sen. Kamala Harris Kamala HarrisButtigieg stands in as Pence for Harris's debate practice First presidential debate to cover coronavirus, Supreme Court Harris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda MORE (Calif.) said firing the special counsel would be an impeachable offense.

“I can't see how it wouldn't be,” she told constituents.

Harris, 53, repeated the sentiment again in Washington this week, saying firing Mueller would “certainly yield impeachment hearings.”

Across the country, when Sen. Elizabeth Warren Elizabeth WarrenHarris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda Judd Gregg: The Kamala threat — the Californiaization of America GOP set to release controversial Biden report MORE was asked at a town hall meeting last week in her home state of Massachusetts last week about impeaching Trump , she also turned to Mueller.

“Right now, I believe it is absolutely critical that the special counsel, Mueller, be allowed to complete his investigation in full with no interference from anyone,” Warren, 68, said.

On Friday, Warren took to Twitter to urge Democrats in the Senate to pass a bipartisan bill “to ensure that Mueller can complete a full, independent investigation.”

“Let’s send a loud, clear message that no one is above the law — not even the President of the United States,” she wrote.

An aide to Sen. Kirsten Gillibrand Kirsten GillibrandSunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Suburban moms are going to decide the 2020 election Jon Stewart urges Congress to help veterans exposed to burn pits MORE (D-N.Y.) also drew the line at Mueller’s firing.

Gillibrand, 51, believes that Mueller “needs to be allowed to do his job without interference from the White House,” the aide said, adding that she backs the bipartisan legislation meant to offer some protection for the special counsel.

“When the investigation is complete, Congress will need to do its job based on the facts,” the aide said.

Offices to a few possible Democratic contenders in 2020, including Sens. Bernie Sanders Bernie SandersButtigieg stands in as Pence for Harris's debate practice Bernie Sanders warns of 'nightmare scenario' if Trump refuses election results Harris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda MORE (I-Vt.), 76; Amy Klobuchar Amy KlobucharSocial media platforms put muscle into National Voter Registration Day Battle lines drawn on precedent in Supreme Court fight Sunday shows - Ruth Bader Ginsburg's death dominates MORE (D-Minn.), 57; and Cory Booker Cory Anthony BookerBipartisan praise pours in after Ginsburg's death DHS opens probe into allegations at Georgia ICE facility Democratic lawmakers call for an investigation into allegations of medical neglect at Georgia ICE facility MORE (D-N.J.), 48, did not respond to a request for comment.

In December Sanders, who ran as a Democrat during the 2016 presidential election, urged Democrats not to “jump the gun” when it comes to impeachment. Sanders said Democrats should wait for the Mueller investigation to be completed before they decide how to move forward on impeachment.

“I think there is a process that has to be followed,” Sanders said on NBC’s "Meet the Press." “I think Mr. Mueller is doing a very good job on this investigation. If Mueller brings forth the clear evidence that there was collusion between the Trump campaign and the Russians, I think you have grounds for impeachment.”

An aide to Sen. Chris Murphy Christopher (Chris) Scott MurphyDemocratic senator calls for 'more flexible' medical supply chain to counter pandemics The Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon GOP chairman to release interim report on Biden probe 'in about a week' MORE (D-Conn.), 44, said he had not weighed in on the matter.

The remarks from Harris, Warren and Gillibrand could put pressure on Democratic leaders in Congress on the issue of impeachment.

Democratic leaders have been careful to not talk up the issue, worrying it could backfire ahead of midterm elections in which the party is growing more and more confident that it could win the House majority.

Doing so would immediately make impeachment more possible, as Democrats would only need a majority vote in the House. In the Senate, a two-thirds vote is necessary for conviction, which would be a higher bar.

Republicans are telling voters that Democrats are getting ready to impeach Trump, partly to energize the GOP base ahead of the election. They see the argument as an effective one to bring voters to the polls.

“If Democrats gain control of the House in November, I have no doubt they will begin impeachment proceedings,” said Republican strategist Alice Stewart. “They want nothing more than to see President Trump removed from office.”

But Stewart said Democrats are clearly walking a fine line as to not appear overtly political.

“The problem with telegraphing it early is that it shows swing voters that Democrats can’t be trusted to govern or rise about the dysfunction,” she said.

In December, about four dozen Democrats voted to take up a resolution by 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) to impeach Trump but it was shot down by a 364-58 vote.

Tom Steyer, the Democratic activist and billionaire, has spent approximately $40 million on ads pushing for impeachment.

Democratic leaders, however, have consistently resisted the efforts.

“It doesn’t do us any favors to be out there pushing for impeachment,” said one Democratic strategist. “It’s more helpful for the Mueller investigation to play out and then we can take it from there based on the findings.”

In a tweet earlier this week, David Axelrod, who served as chief strategist to former President Obama, echoed a similar sentiment. “Dems should NOT commit to impeachment unless & until there’s a demonstrable case for one. It is not just a matter of politics. It’s a matter of principle. If we normalize impeachment as a political tool, it will be another hammer blow to our Democracy.”

In an interview with CNN's Anderson Cooper on Thursday night, Rep. Jerrold Nadler (D-N.Y.) said Democrats in general should not run on the idea of impeaching Trump.

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“It would not be the right thing to do,” Nadler said.

Stewart said she attended the small National Republican Congressional Committee dinner where Chairman Steve Stivers Steven (Steve) Ernst StiversBusiness groups back pandemic insurance bill modeled on post-9/11 law National Retail Federation hosts virtual 'store tours' for lawmakers amid coronavirus Stronger patent rights would help promote US technological leadership MORE (R-Ohio) addressed the threat of impeachment against Trump, as reported this week by The New York Times.

“I agree with his statement that this issue fires up the party base and is beneficial for us to remind voters of this through the midterms,” she said.