"Let's face it, this president has made a mess of foreign policy. North Korea, they're much stronger than they were when he started. In Syria, he messed up, every encounter he has with (Russian President Vladimir) Putin he loses," Senate Minority Leader Chuck Schumer said on ABC's "This Week."
"So I am really worried," he added.
Soleimani, the leader of Iran's elite Quds Force, was killed Thursday in an U.S. airstrike in Iraq, carried out at President Donald Trump's direction. The Pentagon has already announced that it would deploy 3,500 troops to the region in response to rising tensions.
While the Trump administration has argued that it had the authority to carry out this strike, Schumer said on Sunday he didn't believe the president has the authority to go to war in Iraq without congressional approval.
"I think Congress and I will do everything I can to assert our authority," he said. "We do not need this president either bumbling or impulsively getting us into a major war."
"McConnell will not go for a fair trial. You can't have a fair trial without witnesses and documents, particularly those who were right at the scene of the charges," he said.
"I hope, pray and believe there's a decent chance that four Republicans will join us," Schumer said. "If they do we will have a fair trial."
He went on to say that senators voting for witnesses would not necessarily vote to convict the president.
Democratic and Republican leaders have been at an impasse over the rules governing a Senate impeachment trial. House Speaker Nancy Pelosi has said she won't transmit the articles of impeachment to the Senate until she knows what those rules will be.
Recently two moderate Republicans have echoed calls for witnesses and some of Democrats' criticisms.
Sen. Susan Collins, R-Maine, said she was "open to witnesses" on Tuesday in an interview with Maine Public Radio. However, she qualified that support by saying the issue should be decided after the trial has started.
Alaska Sen. Lisa Murkowski said last week that she was "disturbed" by McConnell saying that he would coordinate with the White House on the Senate impeachment trial.
On "This Week," Schumer doubled down on his criticism of McConnell for saying he would coordinate with the White House. He defended himself from McConnell's criticism that Senate Democrats are working with Speaker Nancy Pelosi on impeachment.
"There's always consultation, people can discuss things," Schumer said. " He said he's taking his orders from the White House. I'm not taking -- we Senate Democrats are not taking our orders from anybody."
Democrats have argued that emails leaked on Thursday bolster their case for having witnesses testify in the Senate.
Those emails include one from August in which White House budget official Michael Duffey told the Pentagon the freeze on aid to Ukraine was at the "clear direction of POTUS."
Schumer said on "This Week" Sunday that he supports Pelosi's decision, adding that the decision to delay sending the articles has allowed for more information to come out.
"In the last two weeks, where we haven't had the articles, lots of new evidence that bolsters our case for witnesses and documents has come out," he said.
"The bottom line is very simple," Schumer added. "We need the truth, not a cover-up, not a sham, not to have some nationally televised mock trial where there's no evidence."