Sen. Elizabeth Warren tweeted that although Soleimani was responsible for the loss of American lives, "this reckless move escalates the situation with Iran and increases the likelihood of more deaths and new Middle East conflict. Our priority must be to avoid another costly war."

Sen. Cory Booker said Soleimani "is a person with American blood on his hands. He's plotted, planned and executed attacks on American citizens, American soldiers. He's been doing much to destabilize that region, destabilize Iraq."

He added, "This has been a deteriorating situation: A president that does not have a clear-eyed larger strategy to not only bring stabilization to the area but make Americans more safe."

Former Vice President Joe Biden. | Scott Eisen/Getty Images

Author and entrepreneur Marianne Williamson tweeted that the killing "was one of the most reckless irresponsible actions ever directed by a US President."

Soleimani was the leader of Iran's elite Quds force of the Iranian Revolutionary Guard, which the Trump administration views as a terrorist organization. The Pentagon said President Donald Trump ordered the strike and that Soleimani "was actively developing plans to attack American diplomats and service members in Iraq and throughout the region."

While a number of Trump's Republican allies were quick to praise the action as taking down a threat to U.S. lives, the move shocked longtime observers of Iran and Soleimani, who predicted it would cause fierce backlash from Tehran. Rep. Justin Amash of Michigan, who defected from the Republican Party over his objections to Trump, tweeted soon after emphasizing that the power to declare war lies with Congress.

Sen. Lindsey Graham (R-S.C.), a hawkish ally of the president, tweeted that Soleimani's death was an appropriate response to Iranian provocation in the region, saying the general had "American blood on his hands" while warning of further U.S. action in the event of Iranian retaliation.

Trump was largely silent as the reactions played out, tweeting only a pixelated image of the American flag.