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2020 Democrats are ready to follow Sen. Elizabeth Warren's (D-Mass.) lead — or perhaps Special Counsel Robert Mueller's.

Warren became the first presidential candidate to call for President Trump's impeachment about a month ago, with more contenders jumping on the train in the weeks since. But now, with Mueller's Wednesday reminder that indicting Trump was "not an option" under Department of Justice guidelines, it seems they're all inching toward the resistance's side.

After Mueller made his first public comments about his Russia investigation, Warren quickly tweeted to characterize it as "an impeachment referral." Sen. Kamala Harris (D-Calif.) and South Bend, Indiana Mayor Pete Buttigieg used that same term in their own Wednesday statements.

Former HUD Secretary Julián Castro, Rep. Seth Moulton (D-Mass.), and former Rep. Beto O'Rourke (D-Texas) all renewed their pleas for impeachment on Wednesday as well, while Sen. Cory Booker (D-N.J.) made his for the first time. Rep. Eric Swalwell (D-Calif.), though, only called for impeaching Attorney General William Barr — perhaps a sign of his allegiance to House Speaker Nancy Pelosi (D-Calif.).

Sen. Bernie Sanders (I-Vt.) has only gone so far as to say it "may be time" for the House to start an impeachment inquiry into Trump. Former Vice President Joe Biden has said that if Trump blocked further Congressional probes into his behavior, then Congress would have "no alternative" but to impeach him. Both frontrunners remained silent on Wednesday.