House Intelligence Committee Chairman Adam Schiff Adam Bennett SchiffTop Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence Overnight Defense: Top admiral says 'no condition' where US should conduct nuclear test 'at this time' | Intelligence chief says Congress will get some in-person election security briefings Overnight Defense: House to vote on military justice bill spurred by Vanessa Guillén death | Biden courts veterans after Trump's military controversies MORE (D-Calif.), who has taken a leading role in the Senate impeachment trial of President Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE, said Sunday that a tweet from the president is "intended to be" a threat.

"I don't think it was personal to refer to the CBS story. What may be personal, though, and I think I have to be very candid about this, is I made the argument that it's going to require moral courage to stand up to this president," Schiff said on NBC's "Meet the Press."

Schiff, the lead House impeachment manager in the Senate trial, added that Trump is a "vindictive" president.

"I don't think there's any doubt about it, and if you think there is, look at the president's tweets about me today saying that I should 'pay a price,'" Schiff said.

WATCH: @RepAdamSchiff: "Look at the president's tweets about me today saying I should pay a price.."@chucktodd: "Do you take that as a threat?"@RepAdamSchiff: "I think it is intended to be." pic.twitter.com/8uWXRSJKj0 — Meet the Press (@MeetThePress) January 26, 2020

"Do you take that as a threat?" NBC's Chuck Todd Charles (Chuck) David ToddSunday shows - Trump team defends coronavirus response Strzok: 'I continue to believe that Donald Trump is compromised by the Russians' GOP chair defends Trump messaging on masks: 'To say that he should have known then what we know now isn't really fair' MORE asked.

"I think it's intended to be," Schiff responded.

Trump tweeted early Sunday morning that Schiff, whom he called a "CORRUPT POLITICIAN," has "not paid the price, yet, for what he has done to our Country!"

The tweet came after Trump's defense team attacked Schiff's credibility during its opening arguments in the Senate proceedings on Saturday.

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Schiff's decision last week to reference a CBS News report that Republican senators' heads would be on a "pike" if they break with Trump in the impeachment fight sparked backlash from some senators, including a few moderate Republicans who could be swing votes to allow witnesses as part of the trial in the upper chamber.

Schiff said Sunday he wants to speak "candidly" about the difficulty GOP senators would face in breaking from Trump but added that he doesn't want to do it in a way that is "offensive" to them.

"It is going to be very difficult for some of these senators to stand up to this president. It really is. There's just no question about it," Schiff said. "I want to acknowledge that, and I don't want to acknowledge it in a way that is offensive to them, but I do want to speak candidly about it."

Schiff added that "if this weren't an issue," senators would not have a problem with calling witnesses.

"If we can't even get the senators to agree to call witnesses in a trial, it shows you just how difficult that moral courage is," he said.

Senators will likely vote early this week on whether to compel witnesses and documents as part of the ongoing trial. If all Democrats vote for the measure, they will need at least four Republicans to join them to be successful.

Trump appeared to be listening to Schiff's interview, tweeting Sunday as it aired in another attack on Schiff's credibility.

"After having been exposed as a fraud and corrupt, can anyone, including Sleepyeyes Chuck Todd of Fake @NBCNews, continue to listen to his con?" Trump tweeted.