Presidential candidates and congressional leaders said Sunday they were stunned and heartbroken by news of the mass shooting at Orlando's Pulse nightclub.

Presumptive Republican nominee Donald Trump reacted to the shooting by attacking presumptive Democratic nominee Hillary Clinton and President Obama.

"In his remarks today, President Obama disgracefully refused to even say the words 'Radical Islam,' " Trump said in a statement. "For that reason alone, he should step down. If Hillary Clinton, after this attack, still cannot say the two words 'Radical Islam' she should get out of this race for the presidency."

Clinton said the country needs to "redouble our efforts to defend our country from threats at home and abroad."

"That means defeating international terror groups, working with allies and partners to go after them wherever they are, countering their attempts to recruit people here and everywhere, and hardening our defenses at home," she said. "It also means refusing to be intimidated and staying true to our values."

Sen. Bernie Sanders of Vermont, who remains in the race for the Democratic nomination, called the shooting "horrific" and "unthinkable."

"We should not be selling automatic weapons which are designed to kill people," Sanders said on NBC's Meet the Press. "We have got to do everything that we can on top of that to make sure that guns do not fall into the hands of people who should not have them, criminals, people who are mentally ill. So that struggle continues."

Congressional leaders also reacted with horror to the shooting that authorities say left 50 dead.

House Homeland Security Chairman Michael McCaul, R-Texas, said "this appears to be the worst terrorist attack on American soil since 9/11."

"And it is a sobering reminder that radical Islamists are targeting our country and our way of life," McCaul said. "But Americans will not be intimidated by these fanatics. Instead, tragedies like we saw in Orlando only strengthen our resolve to fight back against terror and prevail over extremism wherever it emerges."

Senate Minority Leader Harry Reid, D-Nev., said "the entire world stands with the victims and their families."

"While we do not have all the facts yet, one thing is clear: This was a senseless, hateful attack on innocent people, and we must do everything possible to ensure that justice is done," Reid said. "That this act of hate occurred at an LGBT mainstay during LGBT Pride Month makes it all the more horrific. Hatred, terror and bigotry have no place in our country, and we will not allow our nation to be ruled by fear."

Senate Majority Leader Mitch McConnell, R-Ky., thanked the "citizens and first responders who helped rescue and save lives amidst horror and chaos."

"We will continue to monitor developments from local law enforcement and the FBI to determine the exact nature of this crime and whether it was connected to international terrorist groups," McConnell said.

House Minority Leader Nancy Pelosi, D-Calif., said "our hearts ache for all those killed in this senseless attack, and we pray for the swift recovery of all those who were wounded."

"While many questions have yet to be answered, the pain of this attack in a mainstay of the Orlando LGBT community is surely magnified as our nation celebrates LGBT Pride month," Pelosi said. "We will not allow hate and terror to succeed in blinding us with fear."

House Speaker Paul Ryan, R-Wis., wrote on Twitter that he is praying "for those brutally attacked in Orlando."

"While we must learn more about the attacker, the victims and families will not be forgotten," Ryan tweeted.

Rep. Adam Schiff, D-Calif., the senior Democrat on the House Intelligence Committee, said he was told by police that the shooter "declared his allegiance to ISIS (the Islamic State)."

"The fact that this shooting took place during Ramadan and that ISIS leadership in Raqqa has been urging attacks during this time, that the target was an LGBT night club during (LGBT) Pride (month) and, if accurate, that according to local law enforcement the shooter declared his allegiance to ISIS, indicates an ISIS-inspired act of terrorism," Schiff said. "Whether this attack was also ISIS-directed, remains to be determined. I’m confident that we will know much more in the coming hours and days."

Sen. Ron Johnson, R-Wis., chairman of the Senate Homeland Security and Governmental Affairs Committee, said his committee "will work to support the federal role in investigating this terror attack and protecting against further threats."

"As Americans we must unite to defeat terrorism’s threat to our nation's security," he said.

Sen. Ben Sasse, R-Neb., who also serves on the Senate Homeland Security Committee, said the attack may be "the latest front" for Islamic extremists.

"We know that violent Islam is bent on turning American cities into war zones and, given comments from the FBI, it appears that Orlando could be the latest front," Sasse said.

Sen. Marco Rubio, R-Fla., appealed for blood donations from Orlando-area residents to help shooting victims.

"Our prayers are with those injured and killed early this morning in horrifying act of terror in Orlando," Rubio wrote on Twitter.

Members of the Congressional LGBT Equality Caucus today said they were horrified by the shooting.

"Though details are still emerging, an attack during Pride Month against Pulse, an iconic gathering place for LGBT Floridians, has a particularly insidious impact on our entire community," said Roddy Flynn, the group's executive director. "Our thoughts and prayers are with everyone affected by this tragedy."