"Tone it down!" That was the urgent plea from Republican Congressman Mike Kelly as he grappled with the aftermath of the assassination attempt on Donald Trump during a political rally in Butler Farm, Trump's hometown.

"I am bewildered by what has happened to the United States of America," Kelly said in an interview with The Associated Press (AP) on Sunday.

The shocking attempt on Trump's life has starkly highlighted the toxic climate in America's political landscape. While the shooter's motives remain unclear, the violence underscores how behaviors once deemed unacceptable, if not unthinkable, have sadly become all too frequent in American society.

As the 2024 election enters a crucial phase ahead of the national conventions, how the nation responds will test the first presidential contest since 2020, an election that became defined by efforts to overturn Trump's defeat and the Jan. 6, 2021, attack on the U.S. Capitol.

On Sunday, civic leaders, pastors and elected officials from President Joe Biden on down appealed to Americans for unity, urging an end to vitriol.

"We can’t allow this violence to be normalized," Biden said in an evening address to the nation from the Oval Office.

Under a charged atmosphere, the Republican National Convention opens this week in Milwaukee to renominate Trump to lead the ticket, while Democrats prepare for their own convention next month uncertain if the party will stick with incumbent Biden in an expected rematch.

Trump's rhetoric, though tempered in the immediate aftermath of the shooting, had taken on deeper and darker tones in this, his third campaign for the White House.

This spring, Trump, who has accused migrants of "poisoning the blood of the country" and vowed to launch the largest domestic deportation operation, told autoworkers there would be a "bloodbath" in this country if he is not reelected.

"If we don’t win, I think our country is finished," he said during the New Hampshire primary.

Trump has promised retribution on his political rivals, particularly those in the Justice Department after he was indicted on federal charges of storing classified documents at his Mar-a-Lago home and in the conspiracy to overturn the 2020 election.

Trump also made light of violence. When Nancy Pelosi’s husband, Paul Pelosi, was attacked by an intruder looking for the former House speaker at the family’s San Francisco home in 2022 – beaten over the head with a hammer – Trump mocked the security fencing she had installed as insufficient.

Trump drew chuckles in a speech before California Republicans last year when he asked, "How’s her husband doing, by the way?"

Biden, in turn, has warned that Trump's return to power poses a grave threat to the country's civic traditions. He chose a location near Valley Forge, Pennsylvania, for his initial 2024 campaign event, portraying the likely rematch as "all about" whether democracy can survive.

Addressing the nation Sunday, Biden pointed to past examples of political upheaval, including Jan. 6 and more recently harassment of election workers, and said, "There’s no place in America for this kind of violence, for any violence, ever."

Still, one of Trump's potential vice-presidential picks, Senator JD Vance of Ohio, said on social media over the weekend that Biden's earlier rhetoric against Trump "led directly" to the attempted assassination.

And House Speaker Mike Johnson, who said it’s time to "turn the temperature down in this country," also singled out for blame Biden’s recent comments during a call with political donors in which the president said, "It’s time to put Trump in the bullseye."

Johnson said he knows Biden didn’t literally mean Trump should be targeted but added, "That kind of language on either side should be called out."

Nick Beauchamp, an associate professor of political science at Boston’s Northeastern University, said there is an opportunity now for political leaders to "start framing their critiques of the others in words that explicitly denounce violence."

From the 1968 killings of American leaders Bobby Kennedy and Martin Luther King Jr. to the 1981 attack on President Ronald Reagan to shootings of Republicans and Democrats in the past decade, the violent strain has always been part of American politics.

Other violent incidents have intersected more recently with the nation's political struggles in frightful ways.

Outside Supreme Court Justice Brett Kavanaugh's suburban home, a man with a knife and gun who threatened to kill the justice was arrested in 2022. Members of Congress have experienced increased security threats. And harassment against election officials from cities and states across the nation has led to a wave of departures because of threats to their livelihoods.

Last summer, FBI agents fatally shot a Utah man who had threatened to assassinate Biden and had referred to himself as a "MAGA Trumper." That followed a series of drive-by shootings earlier in the year targeting Democrats in New Mexico, a startling outburst that led to criminal charges against a failed state legislative candidate who had parroted Trump’s rigged election rhetoric.

A gunman who died in a shootout in 2022 after trying to get inside the FBI’s Cincinnati office apparently went on social media and called for federal agents to be killed "on sight" following the search at former Trump’s Mar-a-Lago estate.

Jacob Ware, a research fellow at the Council on Foreign Relations who focuses on domestic terrorism, said, "The warning lights have been blinking red regarding violence in this election cycle for months if not years now."

As Trump took the stage Saturday evening, he had opened the rally in Pennsylvania as he often does, marveling at the "big beautiful crowd" gathered to see him – and demeaning Biden’s own crowds as paltry in comparison.

The former president had just started his speech, launching into his mass deportation agenda and complaints of a nation in decline.

"Our country is going to hell," Trump said.

Minutes later, shots rang out.

Rep. Dan Meuser of Pennsylvania, who was sitting with other Republican officials behind Trump, called it all just a terrible tragedy. "The level of lack of civility and hostility, maybe this will send a ringing signal to all those to cool it," he told the AP.

As Americans took stock Sunday, the common message was a call for unity.

The Rev. Chris Morgan, senior pastor of Christ United Methodist Church in Bethel Park, which is a few streets away from where the shooter lived, urged his congregation during a morning service to pray for the country.

"Clearly there’s a lot going on and a lot that is causing people to have great anxiety and great struggle," he said. "I want to encourage you to be praying for those that have been involved that they too can find what it means to show kindness to others."