MILWAUKEE—The nation’s political contrast could hardly be more stark: images of a defiant and bloodied Donald Trump surviving an assassination attempt cast against persistent questions about President Biden’s mental sharpness and ability to stay in the race.

In an already turbulent contest, the jarring chaos witnessed in western Pennsylvania Saturday night—and the fear, anger and images it generated —are certain to alter the contours of a campaign where even small shifts of voters could prove decisive.

As the GOP descended on Milwaukee for this week’s Republican National Convention , the former president’s supporters grew more vocal than ever in condemning political attacks against him, saying he literally has taken a bullet for the American people. Republicans who had rallied behind Trump as criminal indictments piled up appeared to become even more fervent in their support in the hours after the shooting. Democrats were mostly reserved, saying little about the race beyond condemning the shooting and calling for calm.

“Before yesterday, Joe Biden ’s chances of re-election were slim. After yesterday they are virtually nonexistent,” Republican pollster Whit Ayres asserted, suggesting Democrats would likely focus instead on maintaining some control in Congress. Similarly, GOP strategist Dave Carney said the attack on Trump “will energize his base and casual voters and takes the knees out of team Biden’s singular theme of preserving democracy.”

But Democratic pollster Mike Lux didn’t see it that way. “I don’t see it changing the fundamentals of the race. It would be nice if it meant everybody would calm their rhetoric down,” he said.

He added that his advice to Biden’s campaign would emphasize policy-focused contrasts. “I would refocus the messaging for a while at least on economic policy,” Lux said. “And I would refocus it on reproductive rights and do less of ‘Trump’s a danger.’”

Images of Trump, fist pumping in the air against an American flag backdrop and blood streaming across his face, have circulated widely on social media, with Republicans characterizing him as a defender of freedom. One of the images has already been previewed as a coming cover photo in Time magazine.

Many of Trump’s campaign surrogates took to social media, calling on Biden and others to lower the political temperature—even as some Republicans also sought to blame Democrats for the tragedy—and suggested he drop all federal criminal charges against Trump as a means of doing so.

Trump on Sunday said it was “God alone” who saved him, and called for unity.

“In this moment, it is more important than ever that we stand United, and show our True Character as Americans, remaining Strong and Determined, and not allowing Evil to Win,” he wrote on his social-media platform.

Nikki Haley , Trump’s final major challenger in the presidential primary and a persistent critic, now plans to speak at the GOP convention following the shooting. She wasn’t included in the original program, and a representative for Haley previously said she wasn’t invited. She is expected to speak Tuesday, according to a person familiar with her schedule.

“I think now more than ever a call for unity across the board is needed,” said Ozzie Palomo, a Republican bundler who was fundraising for Haley but is now supporting Trump. “As Trump’s last primary opponent, if Nikki’s message is clear that the party and country need to come together, it’s a positive for Trump and for the party.”

Some major business figures have also rallied behind Trump. Billionaire entrepreneur Elon Musk and hedge-fund manager Bill Ackman formally threw their support behind the Republican in the wake of the shooting.

While the race remains close nationally , Trump, 78, has led in most crucial battleground state polls, amid calls for Biden to consider stepping down and allowing someone else to lead the ticket following his shaky performance at their first debate on June 27.

Biden’s campaign suspended political advertising shortly after the shooting. The president was attending a church service in Rehoboth Beach, Del., when gunfire rang out at the Trump rally, and he briefly addressed reporters traveling with him Saturday night calling for unity. He cut short his weekend plans on the shore, returning to the White House late Saturday night hours after the shooting.

Sunday evening, Biden gave brief remarks from the Oval Office, urging Americans to reflect on their political rhetoric. The assassination attempt “calls on all of us to take a step back—take stock of where we are, how we go forward from here,” Biden said.

“You know the political rhetoric in this country has gotten very heated. It’s time to cool it down,” Biden said. “Politics must never be a literal battlefield—God forbid a killingfield.”

Biden called Trump following the incident, according to the White House. A source briefed on the discussion described the call as “short and respectful.” Biden also postponed a speech on civil rights he had planned to give Monday at the LBJ Presidential Library in Austin, Texas.

Democrats’ rhetoric on the campaign trail had become increasingly aggressive since the debate. Speaking to donors in a July 8 call, Biden said: “It’s time to put Trump in the bull’s-eye,” saying that the Republican had been mostly out of view since the debate and that his controversial positions needed more attention. The Biden campaign sent the quote to journalists after the call, a sign that the Biden team viewed the comment as part of their message rather than a gaffe.

Republicans highlighted and criticized the “bull’s-eye” quote following Saturday’s shooting. The Biden campaign didn’t respond to a request for comment about the quote Sunday.

The president’s campaign has been making the case that Trump is a threat to democracy, particularly in light of his effort to overturn the 2020 election and the Jan. 6, 2021, riots at the Capitol by his supporters. Trump has in the past also employed dark and violent rhetoric .

In a campaign swing last weekend through Pennsylvania with Biden, Rep. Madeleine Dean (D., Pa.) said that Trump is “corrosive to our democracy.” Also at the campaign stop, Sen. John Fetterman (D., Pa.) said, “If it wasn’t for Joe Biden, Trump would be at the White House and he’d be campaigning for his third term.”

But some Republicans called for Democrats to tone down their assertions about Trump.


“We were one inch from an absolute cataclysm in this country. Thank God it didn’t happen,” said GOP strategist Scott Jennings. “Maybe—just maybe—some folks can take a break from outrageously describing political opponents as Hitler, dictators, causing bloodbaths, and existing as a literal threat to democracy who will bring about the end of our nation should they win an election.”

Biden, 81, is facing a revolt within his own party that has unspooled over the past two weeks since the debate, with 19 Democratic House members and one senator publicly asking him to end his 2024 presidential campaign because of his poor performance and the questions it raised about his ability to effectively make a case against Trump. Others have privately made similar comments.

The intense media spotlight that had been on Biden’s age has now shifted to Saturday’s shooting, and it isn’t clear whether the steady trickle of Democratic defections away from Biden will continue without that pressure.

The former president arrived in New Jersey late Saturday to return to his home in Bedminster, his intended plan after the rally. Video of his arrival showed Trump slowly descending from his Trump-branded plane. He waved to the cameras briefly. His wound wasn’t visible.

Trump said Sunday that he still plans to attend the convention, set to begin here on Monday. But the shooting is certain to alter the message and tone of the gathering. Security, already at high levels, is likely to be even tighter.

The former president is expected to announce his vice presidential running mate during the convention, which culminates Thursday evening with a nomination acceptance speech from the former president.

The extent to which voters will rally around Trump is difficult to predict. When President Ronald Reagan survived an assassination attempt in March 1981, his job approval rating in Gallup polls rose modestly. But within a few months his standing was roughly back where it had been before the incident.

Republican strategist Frank Luntz, writing on social media , suggested the shooting was most likely to affect turnout among voters on the right. undefined undefined “In the end, voters will settle down and return to their candidate of choice. The people who move towards Trump out of sympathy will probably move back,” he wrote. “But what happened in PA will definitely impact the final vote, guaranteeing that every Trump voter will actually vote. Biden, or any other candidate the Democrats put up, will not have the same participation certainty.”

Nathan Gonzales, the editor and publisher of Inside Elections, cautioned that “the worst time to analyze the political fallout from an event is during it or in the immediate aftermath, and that’s where we are.”

“It’s going to take at least a few days, if not a couple weeks to really get a sense on how, or if, this historic event changed a largely stable race,” he added. “There can be a difference between a historic event and a game-changing event.”

Write to Vivian Salama at vivian.salama@wsj.com , John McCormick at mccormick.john@wsj.com , Annie Linskey at annie.linskey@wsj.com and Alex Leary at alex.leary@wsj.com