"If the Senate Republican majority refuses to discipline him through impeachment, he will be unbounded," Coons tells "Meet the Press."
"The only reason that Speaker 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 during an exclusive interview on "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," the Delaware senator continued. "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 against Trump in a party-line vote. The articles allege that Trump abused his power with an attempt to pressure Ukraine to open an investigation into a political rival, former Vice President Joe Biden, and that he obstructed Congress' investigation into those allegations.
The full House is expected to vote on them this week. If those articles pass, as expected, it will be up to the Senate to hold a trial and ultimately vote on whether to remove Trump from office, which would require support from two-thirds of the GOP-controlled Senate.
Coons, a member of the Senate Judiciary Committee, called on Senate Majority Leader Mitch McConnell to work with Democrats to make sure the allegations get a fair shake in the House.
"The American people deserve the truth, not political theater," he said.
Also appearing on "Meet the Press," Sen. Patrick Toomey, R-Pa., said that McConnell has to be sure the president receives a fair trial.
But he said that the Senate should be deliberative about hearing the allegations against Trump. "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 noted there "might be a lot of agreement on the facts" between Republicans and Democrats on the matter, but added there is "a big disagreement about what rises to the level of impeachment."