Sen. John Fetterman, D-Pa., says the U.S. has to "turn down … the temperature" following President Trump's attempted assassination at a rally on Saturday. 

The senator referred to the shooting as a "dark day" and "tragedy" during an interview with CNN's "State of the Union" on Sunday morning. 

Fetterman said it might be necessary for Congress to investigate what, if anything, went wrong from a security standpoint during the Trump rally on Saturday. 

"We can't assume that these kind of things can't happen again," Fetterman said. "We just have to turn down … the temperature on this. And this election is going to be the biggest kind of election in our lifetime, and we have months ahead of us."

He added: I'm not gonna make it about politics."

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The Democratic Senator said he was in the region near the outdoor rally venue in Butler, Pennsylvania, when it happened Saturday evening, to attend a vigil for a transgender teenager who had been murdered.

Americans fear rising political violence, recent Reuters/Ipsos polling shows, with two out of three respondents to a May survey saying they worried that violence could follow the election.

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Fetterman said he is grateful that Trump is making a full recovery, is thinking about the rally attendee killed and hopes the two people who were critically injured will be okay. 

"My drive back home last night from northwestern Pennsylvania … I was just incredibly sad," he said. "I just can't believe that this is the place where our nation is."

Reuters contributed to this report.