Adam Kinzinger reiterated his support for Kamala Harris in the US presidential election at the Toronto film festival on Saturday, but warned that there may be more eruptions of violence should she win.

The former Republican congressman, whose party turned against him when he voted to impeach former president Donald Trump after the January 6 insurrection, was speaking to an audience following the world premiere of The Last Republican.

The crowd-pleasing documentary, with healthy doses of comic relief in its coverage of outrageous and tragic political events, follows Kinzinger for over a year as he endures the fallout from his efforts to hold Trump accountable for inciting the riot as part of the United States House select committee on the attack. The film is a portrait focusing on the costly personal sacrifice to do what both Kinzinger and the director Steve Pink repeatedly remind is simply the right thing.

After the screening, Kinzinger said history could repeat itself at a time when his party has “lost its mind” but doesn’t believe the violence will play out in exactly the same way. The battlegrounds won’t be Capitol Hill, according to the politician who recently spoke during a prime slot at the Democratic national convention, but individual states.

“Look at Arizona for instance,” Kinzinger said. “Assume Arizona goes for Kamala. But it’s a Republican legislature. The legislature has to be the one to certify Kamala as the winner. I can see a pressure campaign where these people simply will not vote to certify her the winner. And what happens then? We’re in a constitutional crisis. According to the constitution, if the state legislature decides it’s just going to certify Trump, even if its [voters] went the other way, we have to accept that in the federal government … That’s a real concern I have. You can see violence at these statehouses that don’t have the security we have. Our security got overrun that day for God’s sakes and we have 500 times the security that statehouses do.”

The Last Republican is directed by Steve Pink, a self-described leftie who Kinzinger suspects has contempt for his politics. The film opens with Pink sharing his admiration for Kinzinger’s resolute stand – he was one of 10 Republicans to vote for Trump’s impeachment, and the only one next to Liz Cheney to sit on the January 6 committee. Kinzinger reciprocates, explaining that he’s agreeing to ignore the ideological gap and take part in the film because Pink directed Hot Tub Time Machine, which he loves.

Pink’s first foray into documentary is a handshake between liberal Hollywood and a Republican that occasionally leans into odd couple comedy. The director and his subject rib each other throughout for opposing political beliefs that the film shies away from interrogating. At one point Kinzinger admits his pro-life stance, but his voice wavers a bit, hinting at the slightest opening that he could be swayed. During the same interview, Pink declares: “If this documentary helps you win the presidency and you enact horrible conservative policies, I swear to fucking God!”

His profile on the extremely charming Kinzinger certainly makes the case that the kid who once dressed up as the Illinois governor Jim Edgar for Halloween and grew up practically indoctrinated into Republican politics would have made a decent presidential candidate. The film revisits a heroic act, when Kinzinger, in his 20s, rescued a bleeding woman from an attacker with a knife. The act of self-sacrifice, the film gently suggests, foreshadowed his recent actions.

The Last Republican doesn’t reveal anything particularly new about January 6 and Kinzinger’s work as part of the committee, but forensically revisits the damning moments before and after the attack. Kinzinger reflects on the Republican conference call, when the former House speaker Kevin McCarthy says he would be voting against certifying Joe Biden’s election win. Kinzinger says he warned McCarthy on the call that such an action could lead to violence. McCarthy’s response, which can be heard in the doc, was a dismissive “OK Adam” before he called for the “next question”.

As The Last Republican cycles through testimony, Kinzinger offers personal reflections and feelings about how things happened, describing January 6 as a bad bender that the Republican party should have woken up from and sipped water to cleanse its system and recover. Instead, they backed Donald Trump. “You could always fix a hangover by starting to drink again,” says Kinzinger, tying up the analogy.

Kinzinger expresses that he was angrier at his old friend Kevin McCarthy than Trump. “He’s just nuts,” Kinzinger says of the latter.

He admits he wanted nothing to do with the January 6 committee. “Please dear Jesus not me,” he would say before Nancy Pelosi announced that she would be seeking his participation without calling him first.

Following the screening, Kinzinger tells the audience that almost every Republican congressman knows the 2020 election “wasn’t stolen” and “most of them would tell you that they think Donald Trump is crazy”. He adds that before impeachment, he believed there was going to be 25 votes in favor, instead of just the 10 who did, because many were too scared to take that stand. “I would have people come up to me all the time and say, ‘Thanks for doing it because I’ll lose in my district if I do it, but thank you.’” He’s exasperated by the gall of it.

Kinzinger not only lost his district but was bombarded with hate while ostracized not just from his party but his own extended family. In one scene, his mother Betty Jo Kinzinger recalls a phone call from an old community friend who tells her she doesn’t like Adam any more. “You don’t have to like Adam,” she says, “but you don’t have to tell his mother that.”

In the film, Kinzinger’s staff can be heard sorting through the relentless phone calls to his office, ranging from angry voters to terrifying threats, deciding which calls should be referred to Capitol police. The vitriol is so much that they keep a cabinet near their desk filled top to bottom with what you would think is an apocalyptic supply of Kleenex boxes. The reveal elicited a hearty laugh from the audience. But the trauma behind it is all too real.

“Over time it takes a toll that you don’t recognize on you,” Kinzinger told the audience. He said that the threats we hear in the film aren’t just a tiny sample, reciting one caller who wishes Kinzinger’s son, who was six months old at the time, would wander into traffic and die.

“The people that call the death threats are probably not the ones that are going to come,” Kinzinger continued, who says he was swatted just a week before, a common occurrence when he speaks out. “The ones that are going to come are not going to let you know ahead of time that they’re going to be there.

“I would always conceal and carry,” Kinzinger continued, “not because I’m just some crazy gun guy. But that was my way to defend myself in security … You’re living with security [with] your work. You always make sure to lock the doors and arm the system at night. But after a while I realized that I’m keeping distance from people. And I don’t want to be that way.”

When pressed about why it’s so hard for his fellow Republicans to question the party line and Trump, Kinzinger said that many were just clinging to what they feel is their identity.

“When you see yourself as a member of Congress,” he said, “and you walk into any room, except the White House, and you’re the most powerful person there, and you have everybody’s attention, it’s really hard to walk away from that … I’ve learned that courage is rare … you have to walk away from your identity. And unfortunately, so many in the Republican party were unwilling and are unwilling to do that.

“Since we filmed this, there have been more people elected into the Republican party that actually are batshit crazy and truly believe some of this. So that’s a scary thing.”

• The Last Republican is screening at the Toronto film festival and will be released at a later date

• This article was amended on 8 September 2024. An earlier version referred to “the House speaker Andrew McCarthy” when it should have said the former House speaker Kevin McCarthy.