"The only reason that Speaker [Nancy] Pelosi changed her position and supported moving ahead with an impeachment inquiry was because what Donald Trump is alleged to have done, and all evidence points to him having done it, which is to invite foreign interference in our next election," Coons said in an interview on NBC's "Meet the Press."
"If he is ultimately exonerated in the Senate, if the Senate Republican majority refuses to discipline him through impeachment, he will be unbounded," Coons said. "And I am gravely concerned about what else he might do between now and the 2020 election — when there are no restrictions on his behavior."
The House Judiciary Committee on Friday approved two articles of impeachment in a party-line vote. The articles allege that Trump abused his power by trying to pressure Ukraine to investigate a political rival, former Vice President Joe Biden, and that he obstructed Congress' investigation of the allegations.
The full House is expected to vote on them this week. If the articles pass, as expected, the Senate would hold a trial and vote on whether to remove Trump from office, which would require support from two-thirds of the Republican-controlled Senate.
Coons, a member of the Judiciary Committee, called on Senate Majority Leader Mitch McConnell, R-Ky., to work with Democrats to make sure the allegations get a fair shake.
"The American people deserve the truth, not political theater," he said.
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Also appearing on "Meet the Press," Sen. Patrick Toomey, R-Pa., said McConnell has to be sure the president receives a fair trial. But he said the Senate should be deliberative about hearing the allegations.
"It's appropriate to make sure the president gets a fair trial here. That's the idea. I think it would be extremely inappropriate to put a bullet in this thing immediately when it comes over," he said.
"I think we ought to hear what the House impeachment managers have to say, give the president's attorneys an opportunity to make a defense, and then make a decision about whether and to what extent we come forward from there."
Toomey said there "might be a lot of agreement on the facts" between Republicans and Democrats, but he said there was "a big disagreement about what rises to the level of impeachment."