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WASHINGTON  — President Barack Obama tried to reassure a nervous nation on Sunday with a plan to defeat the fast-evolving threat of terrorism, as fears multiply of ISIS attacks on the homeland and public trust in his handling of the threat has dipped to record lows.

Obama’s rare 8 p.m. ET Oval Office address reflected growing anxiety that the global showdown with the extremist group has now spread to U.S. soil following a mass shooting in San Bernardino, California, that authorities are treating as terrorism.

Obama called on Congress to take specific steps to help combat terrorism following the terror attack last week in San Bernardino. “Congress should act to make sure no one on a no-fly list is able to buy a gun. What could possibly be the argument for allowing a terror suspect to buy a semi-automatic weapon?” he said.

“This is a matter of national security. We also need to make it harder to buy higher-powered assault rifles like the ones that were used in San Bernardino. What we can do and must do is make it harder for them to kill.” The President said if Congress agrees the U.S. is at war with ISIS, “it should go ahead and vote for the continued use of military force.”

“I think it’s time for Congress to vote to demonstrate that the American people are united and committed to this fight,” he said.

His appearance comes at a time of public disquiet over terrorism and a political debate over the threat, now consuming the 2016 campaign, raging at levels not seen since the aftermath of the September 11 attacks in 2001. Obama has also struggled to convince critics he has a viable strategy for destroying ISIS in the Middle East and has been accused of downplaying the threat from the group for political reasons.

The gravity of the occasion is underscored by Obama’s decision to use the symbolic power of the Oval Office for only the third time in his presidency, following addresses on the Gulf of Mexico oil spill and the end of the Iraq war in 2010.

‘More of what we can expect’

“Unfortunately I think it is more of what we can expect in the future — whether it is copy cats or whether you have, again, either ISIS-aligned or inspired or ISIS-directed individuals,” said Dean Alexander, director of the Homeland Security Research Program at Western Illinois University. “It might instigate ISIS to take a more proactive approach to send individuals here to the U.S. or to try to recruit.”

The California attack is so concerning because Syed Rizwan Farook and Tashfeen Mali, the couple behind it, seem to have been inspired by ISIS, but not linked directly to the group.

Such a “self radicalization” scenario by Muslims on U.S. soil has long been feared by anti-terrorism experts who say such attacks are nearly impossible for the administration to detect and stop.

Still, the White House insists that an intensifying air war against ISIS in Syria and Iraq is succeeding in curtailing the group, and Obama is adamant that sending U.S. ground troops back into a Middle East conflict is not the way to defeat it.

But critics say he has consistently underplayed the threat from ISIS because its rise conflicts with his assurances that he has severely degraded the threat from global terror — a key legacy issue as he nears the end of his second term.

“At every turn, the American people can see with their own eyes how tragically wrong the President has been,” said John Hannah, a former deputy national security advisor for Vice President Dick Cheney.

“Instead of being the JV team, they turn out to be the most powerful terrorist group in history,” said Hannah, who also worked in the Bill Clinton and George H.W. Bush administrations.

“Instead of being contained, they’re conducting or inspiring spectacular terrorist operations on multiple continents — including now in the United States. ISIS affiliates are spreading across the Middle East and south Asia.”

Ground troops

“They can’t beat us on the battlefield, so they try to terrorize us into being afraid, and changing our patterns of behavior, and panicking, and abandoning our allies and partners, and retreating from the world,” Obama said at a press conference in Malaysia last month.

But his critics — like Rubio for instance — warn that the president, and Democratic front-runner Hillary Clinton, are being too politically correct and even refuse to voice the true nature of the threat.

“We’ve got to be clear — we are at war with ISIS. We are at war with radical, apocalyptic jihadists. And that is why ultimately we must defeat them,” Rubio said on Fox News on Friday, as he accused Obama of using the California attack as a new spur for his push for gun control rather than as a terrorist outrage.

New Jersey Gov. Chris Christie rejected the idea that Muslims would be offended by the use of specific terminology to describe the threat.

“Now when you say radical Islamic jihadist, they understand, the rest of the Muslim community understands,” Christie said on “Face the Nation.”

Obama has taken a similar line.

“We must never accept the premise that they put forward because it is a lie,” Obama said in February.

“Nor should we grant these terrorists the religious legitimacy that they seek. They are not religious leaders. They are terrorists.”