In a rare prime-time speech, Obama called the shootings in San Bernardino, California, an "act of terrorism." He prodded Congress to adopt new restrictions on assault weapons and ban people who are on no-fly lists from purchasing firearms.
And he urged Americans against associating terrorism with all Muslims -- even as he said that extremism in some communities is "a real problem that Muslims must confront without excuse."
GOP presidential front-runner Donald Trump, who had promised to live-tweet the speech, came to the quick conclusion, "We need a new President - FAST!"
The real estate mogul had little else to say other than, "Is that all there is?"
In other tweets, Trump excused Obama for reading a prepared speech off of a teleprompter, but said he hoped Obama wouldn't criticize Second Amendment gun rights.
After Obama's speech, Florida Sen. Marco Rubio called Obama "completely overwhelmed" by the terrorism threat in a Fox News appearance.
Rubio also said Obama was "cynical" for spending time urging Americans not to discriminate against Muslims.
"Where is the evidence that we have widespread discrimination against Muslims?" he said.
And he blasted Obama's focus on gun control.
"The notion that a radical jihadist who is on a no-fly list is going to walk into a local gun shop to purchase a gun is absurd," Rubio said.
Republican Ben Carson called Obama's speech "strange."
"President Obama's declaration tonight that his policies are working was strange," Carson said in a statement. "Strange that it took four days from the attack to respond and even more strange that somehow the attack on our soil is proof his policies are working. One must wonder who has contained who."
In a statement, Carson described Obama's call for tougher gun control laws as an "unfortunate" response to the act of terror in San Bernardino.
"Putting aside the absurdity that numerous ordinary, law-abiding Americans are mistakenly included in the no-fly list, we should not that both of the terrorists had a clean record and would not have been affected by the President's proposals to outlaw gun purchases by those on the list.
Texas Sen. Ted Cruz responded to Obama by taking aim at the immigration system.
"If I am elected President, I will direct the Department of Defense to destroy ISIS. And I will shut down the broken immigration system that is letting jihadists into our country," he said in a statement. "Nothing President Obama said tonight will assist in either case."
Former Florida Gov. Jeb Bush called the struggle against ISIS "the war of our time."
"We need to remove the self-imposed constraints President Obama has placed on our intelligence community and military, and we need to put in place an aggressive strategy to defeat ISIS and radical Islamic terrorism as I have proposed," he said.
Bush also said Monday on Fox News that Obama only spoke on the issue because he felt like he was forced to.
"I think he was told to do it because people are feeling anxious, but he didn't lay out anything new and he spent more time talking about what we shouldn't do than what we should."
Kentucky Sen. Rand Paul used his statement to criticize Obama's calls for gun control reforms and also took aim at the U.S. immigration system.
"We should not double down on this failed and dangerous policy that the President called for tonight," he said in a statement. "We must arm our allies, the Kurds, and insist on Arab boots on the ground for our allies in the region."
Ohio Gov. John Kasich, another 2016 contender, said Obama's strategy "is not enough."
"Bolder action across the board is needed because our way of life is what's at stake," said Kasich in a statement. "Also, when terrorists threaten us, our response can't be to target our own constitutional rights. Our rights aren't the problem, our unwillingness to act to defeat extremists is the problem. We need to decisively and aggressively protect our nation and our ideals."
Former Arkansas gov. Mike Huckabee criticized Obama on Monday for taking so long to identify America's war with radical Islam.
"This administration has consistently misread the very obvious signs of radical Islamic Jihadism, and it's just unfortunate that we continue to see more waves of this," Huckabee said in a Fox appearance. "It sounds to me that the President is more interested in protecting the reputation of Islam, than he is in protecting the American people."
Huckabee then went on to describe ISIS as a "malignant tumor."
"If it's allowed to go unchecked, if you don't go in and kill it, destroy it -- not just contain it, destroy it -- it will continue to grow, metastasize, and kill us."
South Carolina Sen. Lindsey Graham said Monday that he would be willing to give Obama as well as the future president "whatever" authority they need to destroy ISIL.
"If I'm not the future president, the next president needs the authority to destroy ISIL," he told CNN. "The president has the right goal, he just doesn't have the right strategy," Graham added.
Graham also said that the president needs more authorization than Obama's last request, which had a three year limit and no ground troops.
"This is not a terrorist organization, this is a terrorist army," Graham said.
Republican National Committee Chairman Reince Priebus also blasted Obama's speech.
"We will defeat ISIS but we cannot do so by continuing the current approach. The path laid out by President Obama and supported by Hillary Clinton has not worked, and ISIS has only gained in strength," Priebus said in a statement. "The attacks in San Bernardino should serve as a wake-up call for Obama and Clinton that the way to victory is not through the status quo but refocusing our efforts to defeat ISIS."
Democratic presidential candidates Hillary Clinton, Bernie Sanders and Martin O'Malley have yet to comment on the President's remarks.