The candidates, vying for the right to challenge Trump in the November 2020 election, questioned whether the president had a broader strategy in dealing with Iran, and used the action to highlight their approach to dealing with foreign adversaries.
"President Trump just tossed a stick of dynamite into a tinderbox," former U.S. Vice President Joe Biden said in a statement.
At a campaign event in Dubuque, Iowa, he added that no American would mourn Soleimani's death but "the prospect of direct conflict with Iran is greater than it has ever been."
Liberal U.S. Senator Bernie Sanders, who has consistently opposed U.S. military intervention overseas, said the move "brings us closer to another disastrous war in the Middle East that could cost countless lives and trillions more dollars."
The overnight attack against the general, regarded as the second most powerful figure in Iran, was a dramatic escalation of hostilities in the Middle East between Iran and the United States and its allies, principally Israel and Saudi Arabia.
U.S. Secretary of State Mike Pompeo said the strike aimed to disrupt an "imminent attack" that would have endangered Americans in the Middle East. But it was a risky gamble for Trump, who has criticized longstanding U.S. entanglements in the region and promised to end "endless wars."
Republicans said the move was a sign Trump - who was impeached by the Democratic-led House of Representatives last month and faces a Senate trial on charges he abused his office and obstructed Congress - was restoring American strength and leadership.
"At a time when the president is under impeachment by the Democrats, there's nothing wrong with him showing strength and resolve in the face of a foreign threat," said Republican strategist Ron Bonjean, who is close to the White House.
Democrats said it was another troubling indication of Trump's erratic approach to foreign policy.
"We're on the brink of yet another war in the Middle East," said liberal U.S. Senator Elizabeth Warren. "We're not here by accident. We're here because a reckless president, his allies and his administration have spent years pushing us here."
Many of the Democratic White House candidates, who will face voters for the first time in a month when Iowa kicks off the state-by-state nominating battle on Feb. 3, pounced on the strike to emphasize their own foreign policy philosophies and credentials.
Biden, a former chairman of the U.S. Senate Foreign Relations Committee who emphasizes his foreign policy experience, released a 30-second online ad on Friday calling Trump "an erratic, unstable president" and portraying himself as "someone tested and trusted around the world."
Sanders mentioned in his statement his 2002 vote against authorizing war in Iraq, which he frequently uses as a contrast to Biden, who backed the war.Biden, Pete Buttigieg, the former mayor of South Bend, Indiana, and other Democrats made clear in their statements that they viewed Soleimani as a threat, but Warren, Sanders and entrepreneur Andrew Yang did not mention the Iranian commander.