WASHINGTON — President Joe Biden appealed Sunday for the country to “unite as one nation” after the attempted assassination of former President Donald Trump and said he was ordering an independent security review of how such an attack could have happened.

Biden delivered short afternoon remarks from the White House after receiving a briefing in the Situation Room on the investigation. He called for a “thorough and swift” review and asked the public not to “make assumptions” about the shooter’s motives or ties.

The president said he has also directed the U.S. Secret Service to review all security measures for the Republican National Convention, which begins Monday in Milwaukee. Hours later, Audrey Gibson-Cicchino, the Secret Service’s coordinator for the convention, said the weekend attack against Trump did not prompt any changes to the agency’s security plan for the event and officials “are fully prepared.”

In his remarks, Biden said “an assassination attempt is contrary to everything we stand for as a nation.”

“It’s not who we are as a nation. It’s not American. And we cannot allow this to happen,” he said. “Unity is the most elusive goal of all, but nothing is more important than that right now.”

Biden planned to deliver extended remarks to the nation Sunday evening in an address from the Oval Office. His campaign said the president would touch on “the need for every American to come together to not just condemn but put to an end to political violence in this country.”

In the meantime, the president said he and first lady Jill Biden were praying for the family of Corey Comperatore, a former fire chief who was shot and killed during the Trump rally Saturday night in Butler, Pennsylvania.

“He was protecting his family from the bullets,” Biden said. “God love him.”

The president also said he’d had a “short but good conversation” with Trump in the hours after the shootings and that he was “sincerely grateful” that the former president is “doing well and recovering.”

Trump, who has called for national resilience since the shooting, posted “UNITE AMERICA!” on his social media account after Biden’s remarks.

Actually achieving unity will be far more challenging, especially in the midst of a bitter presidential campaign. Biden’s team is grappling with how to calibrate the path forward after the weekend attack on the person he is trying to defeat in November’s election.

Biden, who has set out to brand Trump as a dire threat to democracy and the nation’s founding principles, has put a pause on such political messaging. The president also postponed a planned trip Monday to Texas, where he was to speak on the 60th anniversary of the Civil Rights Act at the Lyndon B. Johnson presidential library.

But the White House said Biden will still make a planned trip to Las Vegas, which will include a campaign event Wednesday. Vice President Kamala Harris nonetheless postponed her planned campaign trip Tuesday to Florida, where she had been set to meet with Republican female voters.

Trump, meanwhile, announced that he was moving up plans to go to Milwaukee and would arrive later Sunday at the Republican convention.

The weekend developments were only the latest upheaval in recent weeks.

Biden’s shaky debate performance June 27 so spooked his own party that some top surrogates and donors turned on him, and nearly 20 Democratic members of Congress called on the president to leave the race outright. Facing mounting questions about whether he was fit for a second term, Biden and his top advisers have been scrambling to salvage his campaign by adding events around the country and more aggressively criticizing Trump.

The campaign hopes the evening address Sunday would provide Biden a chance to further drive home his point about the need for unity, but also to demonstrate leadership that could assuage nervous critics within his own party.

“We’ll debate and we’ll disagree, that’s not going to change,” Biden said in his afternoon remarks. “But we’ll not lose sight of who we are as Americans.”

Some Biden critics are calling out the president for comments in a private call last Monday. A person familiar with those remarks said the president was trying to make the point that Trump had gotten away with a light public schedule after last month’s debate while the president himself faced intense scrutiny.

“We’re done talking about the debate,” Biden reportedly said. “It’s time to put Trump in the bull’s-eye. He’s gotten away with doing nothing for the last 10 days except ride around in his golf cart.”