Donald Trump and Hillary Clinton tore up their presidential campaign schedules after the worst mass shooting in US history.
Both offered prayers and support to the victims of the Orlando massacre, but then gave very different responses to the killing of at least 50 people in a gay nightclub by Omar Mateen.
Mrs Clinton postponed her first joint event with President Barack Obama on Wednesday in Green Bay, Wisconsin, because of the Orlando shooting, and Mr Trump cancelled a rally on Monday in Portsmouth, New Hampshire.
He said he was changing the focus of his speech on Monday at Saint Anselm College in New Hampshire from his case against Mrs Clinton to "this terrorist attack, immigration and national security".
Mrs Clinton, the presumptive Democratic nominee, pushed for gun control and reached out to a key constituency - gays and lesbians.
"The gunman attacked an LGBT nightclub during Pride Month. To the LGBT community - please know that you have millions of allies across our country. I am one of them," she said.
She urged action to keep assault weapons out of the hands of "terrorists or other violent criminals".
Mr Trump, the presumptive Republican nominee, also offered words of support.
But he then spent the day congratulating himself apparently for predicting more attacks inside the US.
One of his tweets said: "Appreciate the congrats for being right on radical Islamic terrorism, I don't want congrats, I want toughness & vigilance. We must be smart!"
Appreciate the congrats for being right on radical Islamic terrorism, I don't want congrats, I want toughness & vigilance. We must be smart!— Donald J. Trump (@realDonaldTrump) June 12, 2016
He renewed talk of his plan to ban Muslims from the US for an indeterminate time, and he went after the president as he stepped up to the podium in Washington to address the nation early Sunday afternoon.
Mr Trump tweeted: "Is President Obama going to finally mention the words radical Islamic terrorism? If he doesn't he should immediately resign in disgrace!"
Is President Obama going to finally mention the words radical Islamic terrorism? If he doesn't he should immediately resign in disgrace!— Donald J. Trump (@realDonaldTrump) June 12, 2016
In his address Mr Obama called the tragedy an act of terror and hate. He did not talk about religious extremists, reluctant to inflame a stunned nation already on edge.
He said the FBI would investigate the shootings as terrorism but that the alleged shooter's motivations were unclear, and the US "must spare no effort" to determine whether Mateen had any ties to extremist groups.
Hours later, a law enforcement official confirmed that Mateen had made a 911 call from the club, professing allegiance to the leader of IS, Abu Bakr al-Baghdadi.
Mr Trump noted that he "said this was going to happen" and insisted Mrs Clinton should drop out of the presidential race.