New Jersey Sen. Cory Booker weighed in on the impeachment proceedings against Trump, for the first time calling for the House of Representatives to begin impeachment proceedings.
"I have been asking for Special Counsel Robert Mueller’s testimony, and today he made his views clear," he said in a statement. "Mueller said directly that it wasn’t for lack of evidence that criminal charges weren’t brought against President Trump -- but because of Department of Justice policy. He made clear that it is the role of Congress to evaluate evidence against a sitting President and act accordingly."
He added: "We have one remaining path to ensure justice is served. It is our legal and moral obligation to hold those who have committed crimes accountable. It’s clear that the House must begin impeachment proceedings. No one is above the law."
In calling for the immediate start of impeachment proceedings, California Sen. Kamala Harris joined Booker in shifting towards a far more aggressive stance on impeachment.
"What Robert Mueller basically did was return an impeachment referral," she wrote in a tweet. "Now it is up to Congress to hold this president accountable. We need to start impeachment proceedings. It's our constitutional obligation."
At a gaggle with reporters in South Carolina, Harris added, "I would suggest that he felt the need to make the statements he made today because the president and the president's attorney general have been conflating issues in a way that had the intention of misleading the American public about what's going on," she said. "Let's hear what Bob Mueller has told us, and I would suggest, that what he is telling us that now Congress is going to have to do its job."
Last month she said "I believe Congress should take the steps toward impeachment," at a CNN town hall.
New York Sen. Kirsten Gillibrand joined Booker and Harris Wednesday in more forcefully supporting steps to remove the president.
"From the beginning, I have called for a proper process in order to secure key testimony and information related to the Mueller investigation, so that Congress - as a co-equal branch of government - can fulfill its responsibility to deliver the truth to the American people," she said in a statement. "It's time for Republicans and Democrats to begin impeachment hearings and follow the facts wherever they may lead. We cannot let this president defy basic accountability measures built into our Constitution."
On Thursday morning, former Colorado Gov. John Hickenlooper expressed his changing sentiments on impeachment in the wake of Mueller's statement, moving towards opening an inquiry despite acknowledging that a GOP-controlled Senate would not impeach Trump.
"After listening to Mueller, and I wanted to hear what he had to say, I think of myself as an extreme moderate. But I think he laid the responsibility clearly at the doorstep of Congress," he said on CNN. "I think we have to begin an impeachment inquiry and that doesn't mean we're going to impeach President Trump tomorrow or maybe ever, but I think we do have an obligation to follow where the facts lead and we have to recognize that – I mean, I'm not naive. Mitch McConnell is never going to impeach President Trump."
"We've got to keep our eye on the prize and recognize we've got to beat Trump at the ballot box. But at the same time, we've got to get the real facts of what did happen," he added.
Mueller on Wednesday broke more than two years of silence about his investigation into Russian interference in the 2016 election and whether President Donald Trump obstructed justice, saying he does not intend to testify to Congress and explaining why he did not have the "option" of charging the president with a crime.
He also said that Justice Department policy prohibits charging a sitting president with a crime, calling it "unconstitutional."
Despite the president claiming "Nothing changes from the Mueller Report ... The case is closed," in a tweet after Mueller wrapped his remarks, a chorus of Democratic contenders hoping to replace Trump in the Oval Office sounded off, repeating previous calls for impeachment proceedings to get underway.
Former Vice President Joe Biden's campaign, stopping short of calling for impeachment, said that Congress should "continue to investigate."
"What is truly troubling is that we have seen this President and this Administration engaging in flagrant, open attacks on the rule of law by throwing up roadblocks early in the stages of Congress' investigation," a campaign spokesperson said in a statement to ABC News. "Not only that, President Trump is now directing an extraordinary internal vendetta against law enforcement and intelligence community investigators who were doing their job."
"Vice President Biden agrees with Speaker Pelosi that no one would relish what would certainly be a divisive impeachment process, but that it may be unavoidable if this Administration continues on its path," the spokesperson continued.
Massachusetts Sen. Elizabeth Warren, one of the first of the White House hopefuls to call for Trump's impeachment, asserted on Twitter, "Mueller’s statement makes clear what those who have read his report know: It is an impeachment referral, and it’s up to Congress to act. They should."
Former Texas Congressman Beto O'Rourke wrote, "There must be consequences, accountability, and justice. The only way to ensure that is to begin impeachment proceedings."
Massachusetts Congressman and former Marine Corps officer, Seth Moulton, said in a tweet, "Mueller did his job. Now it’s time to do ours. Impeachment hearings should begin tomorrow."
Obama-era Housing and Urban Development Secretary, Julián Castro, who first signaled his support for Congress to begin an impeachment inquiry on the eve of the release of Mueller's redacted report, tweeted, "Mueller made clear this morning that his investigation now lays at the feet of Congress. No one is above the law—Congress should begin an impeachment inquiry."
Later, he criticized House Democrats for not acting on the issue and counters their position, arguing it is more politically prudent to act on impeachment.
"By not moving forward with impeachment, not only are you letting him get away with these ten different instances of obstruction of justice or attempting to obstruct justice that Mueller pointed out -- which deserve impeachment -- but politically you are also giving him a clean bill of health. So I'm convinced that this really is a mistake that Congressional Democrats are making right now," he said on MSNBC.
But some of the Democratic competitors are taking a more cautious approach, speaking out with tepid statements that leave the question of impeachment on the doorstep of Congress, including New York Mayor Bill de Blasio, Montana Gov. Steve Bullock, South Bend Mayor Pete Buttigieg, former Maryland Congressman John Delaney, Washington Gov. Jay Inslee, Minnesota Sen. Amy Klobuchar, Ohio Congressman Tim Ryan, Vermont Sen. Bernie Sanders, and California Congressman Eric Swalwell, who sits on the House Judiciary Committee.
"Congress must continue to investigate #ConDon & his cronies. I'm focused on beating @realDonaldTrump in 2020. As a New Yorker, I know all of his tricks and am tough enough to take him on," de Blasio wrote.
"Congress should fulfill its constitutionally empowered oversight of this Administration, and the President and his staff have an obligation to be responsive to those investigations," Bullock said in a statement. "As a former Attorney General, I know that no individual is above the law. While Congress exercises its oversight authority, it is important to remember what Americans are focused on. When I speak with voters in Iowa and across the country, I hear concerns about the costs of healthcare, cost of living, and sharing in the opportunity of our country. We should expect our leaders to focus on both."
"This is as close to an impeachment referral as it gets," Buttigieg, who has previously said the president "deserves to be impeached," wrote in a tweet. "Robert Mueller could not clear the president, nor could he charge him — so he has handed the matter to Congress, which alone can act to deliver due process and accountability."
Buttigieg came out more firmly during an interview with MSNBC: "The next move is up to Congress. And if we're ever going to have any kind of due process, any kind of systemic assessment of whether there will be accountability for the president, the DOJ can't deliver. Congress will have to do it."
"I trust @SpeakerPelosi on the best path forward to hold the President accountable for his abuse of power, his welcoming of Russian interference, and his actions to obstruct justice," Delaney tweeted.
Inslee said Trump and Attorney General William Barr lied about the Mueller report and said that Mueller made clear, there was "no exoneration."
Klobuchar also raised impeachment proceedings as an option and signaled her openness to it without endorsing the move.
"[Mueller] should testify before Congress. He is essentially referring his findings to the House for expanded investigation or impeachment hearings -- and it is Congress's constitutional mandate to provide checks and balances," she said in a statement. "As I have said before, impeachment proceedings are one way to investigate and hold this administration accountable if the White House keeps stonewalling."
"It is clear Mueller thinks the President obstructed justice but felt the law prohibited him from charging the President," Ryan said on Twitter. "It’s Congress’ job to make sure we are true to our founding principle that the President is not a King and must answer to the American people."
Sanders continued to hedge on the issue, repeating his earlier positions in a tweet: "Given the reality that we have a president who believes he is above the law, Congress must continue its investigations. If the House Judiciary Committee deems it necessary, I will support their decision to open an impeachment inquiry."
Swalwell called for Barr's impeachment in a series of tweets responding to Mueller's statement.
While the calls for impeachment grow within the Democratic primary field, Michigan Rep. Justin Amash, who has been rumored for a 2020 Liberation Party presidential run and is the only Republican member of Congress to go that far, re-iterated his position again on Wednesday, tweeting, "The ball is in our court, Congress."
ABC News' Molly Nagle and Adam Kelsey contributed to this report.