Washington DC - Joe Biden's presidential campaign was upended by the assassination attempt on Donald Trump, forcing it to dial down attacks yet also buying the president time to deal with his own political woes.

The president's call to "lower the temperature" after the horrific shooting deprives him, for now at least, of his core strategy of assailing his predecessor as a threat to US democracy.

It comes just days after Biden had tried to pivot his campaign back onto his Republican rival, following weeks of turmoil in the Democratic party over his age and health after a disastrous debate performance.

But the question is how long the 81-year-old can afford to go easy on Trump, and he hinted in an Oval Office address focused on unity that he would laser in on the Republican again soon.

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"I'll continue to speak out strongly for our democracy, stand up for our constitution and the rule of law, to call for action at the ballot box, no violence on our streets," said Biden on Sunday.

Biden said in the primetime speech that he had "no doubt they'll criticize" him at the Republican National Convention, which begins Monday, but that "I'll be traveling this week, making the case for our record."

Despite canceling a trip to Texas, Biden is continuing with a planned visit to the battleground state of Nevada, setting up a split-screen with Trump's convention appearances.

In an opinion piece in the Washington Post, political columnist Karen Tumulty wrote that there could "hardly be a worse moment for Biden to be compelled to redraw his strategy against Trump."

Republicans have accused Biden himself of creating the political conditions that led to the attempt to kill his rival Trump – ignoring their own candidate's history of encouraging violence.

They have pointed in particular to Biden's recent remarks saying he wanted to stop talking about the debate crisis and "put Trump in a bullseye."

The Democratic meltdown over Biden's age following the debate had dominated the airwaves for weeks, but with the gunshots on Saturday, the revolt over his candidacy fell abruptly silent.

House Democrat Dean Phillips, who had made a long-shot primary challenge against Biden based on concerns over his age, told the Axios news outlet it would be "unpatriotic and unprincipled" to raise the issue now.

Biden has also sought to strike a presidential tone over the shooting, coming out quickly to react on Saturday and addressing the nation Sunday in just the third Oval Office speech of his presidency.

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The message of unity in appearances like this is not just to Republicans – they are also a signal to Democrats that they need to rally behind him as a leader in a time of crisis.

But if the shooting could unify Democrats, it could also doom Biden's reelection bid, with the president already behind in most polls.

Iconic images of a bloodied Trump waving his fist after the shooting are already galvanizing Republican hopes that voters will further rally behind him for a landslide victory in November.