President Joe Biden speaks, Saturday, July 13, 2024, in Rehoboth Beach, Del., addressing news that gunshots rang out at Republican presidential candidate former President Donald Trump's Pennsylvania campaign rally. Associated Press

WASHINGTON — President Joe Biden on Sunday appealed for the country to “unite as one nation” after the attempted assassination of his predecessor, 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 on the investigation in the Situation Room. He called for a “thorough and swift” review and asked the public not to “make assumptions” about the shooter's motives or affiliations.

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.

“An assassination attempt is contrary to everything we stand for as a nation. Everything. It’s not who we are as a nation. It’s not American. And we cannot allow this to happen,” Biden 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.

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

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

The president also said he'd had a “short but good conversation” with Trump on Saturday evening and that he was sincerely grateful that Trump is “doing well and recovering.”

Trump, who called for national resilience in the hours after the shooting, posted on his social media account after Biden's remarks, “UNITE AMERICA!”

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

Biden, who has set out to brand Trump as a threat to democracy and the nation’s founding principles, has put a pause on such political messaging at least in the short term. In the moments after the shooting, Biden’s reelection campaign said it was putting a hold on “all outbound communications and working to pull down our television ads as quickly as possible.”

The president also postponed a planned trip to Texas on Monday, where he was to speak on the 60th anniversary of the Civil Rights Act at the Lyndon B. Johnson presidential library. An NBC News interview between Biden and anchor Lester Holt on Monday will now occur at the White House, instead of in Texas, as initially planned.

Vice President Kamala Harris is also postponing a planned campaign trip to Florida on Tuesday, where she had been set to meet with Republican women voters.

Trump, meanwhile, announced that he was moving up plans to arrive at the Republican convention, heading for Milwaukee later Sunday, where criticism of Biden and the Democrats is sure to be searing.

The weekend developments were only the latest upheaval in a campaign that has been extraordinarily topsy-turvy in recent weeks.

Biden’s shaky debate performance on 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. 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.

Saturday's attack upended — at least briefly — that counteroffensive on the cusp of the Republican convention.

The campaign hopes the evening address on 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.”

Although investigators are still in the early stages of determining what occurred and why, some Biden critics are calling out the president for telling donors in a private call on Monday that “it’s time to put Trump in the bull's-eye.”

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 from many even within his own party. The person spoke on the condition of anonymity to more freely discuss private conversations.

In the donor call, Biden said: “I have one job and that’s to beat Donald Trump ... I’m absolutely certain I’m the best person to be able to do that.”

Biden continued then: “So, we’re done talking about the debate. 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, bragging about scores he didn’t score … Anyway I won’t get into his golf game.”

Republican presidential candidate former President Donald Trump is helped off the stage by U.S. Secret Service agents at a campaign event in Butler on Saturday. Associated press