WASHINGTON - As investigators search for a motive behind the deadly rampage in San Bernardino, politicians are searching for a way to talk about it.

The details of the California massacre at a holiday party - pointing at a possible link to Islamic militants and raising questions about domestic extremism - quickly knocked both Republicans and Democrats off their talking points, upending what has become a grim and predictable ritual in American politics.

Democrats who have vowed to use every mass shooting as a moment to call for new gun laws were tempering their rallying cries. Republicans who point to mental health services as the solution had begun to blame extremist views.

In an interview Friday morning, Sen. Marco Rubio said "we have a violence problem in America" but added it's not exclusive to guns. A U.S. citizen with a clean background but extremist views is perhaps the most significant threat, he said.

"That is a very significant, difficult threat to confront," the Florida Republican said on CBS This Morning.

Hillary Clinton also moved carefully in that direction. "It's becoming clearer that we are dealing with an act of terrorism," she said Thursday. "It does raise some serious questions about how we need to be protecting ourselves."

Addressing those questions will become a far more complex debate - with fewer clear-cut policy prescriptions - than the well-trod conversation over gun control.

President Obama has said he worries about the difficulties of preventing a homegrown or "lone-wolf" attacker on U.S. soil - and the limits of security measures to prevent them. But proposals for tighter domestic security measures or expanded intelligence gathering powers are politically fraught. For Republicans the issue could become quick campaign fodder - although they risk politicizing a national security threat without offering a clear alternative.

Obama has tried to walk a fine line in the discussion, mindful of an ongoing investigation and shifting circumstances. The White House has deferred to law enforcement and on Friday allowed the FBI to announce the probe had become a terrorism investigation.

Briefing reporters, FBI Director James Comey said the two killers showed signs of radicalization but were not part of a broader network.

There's still "a lot of evidence that doesn't quite make sense," he said.

In his only extended comments on the shooting, the president asked for patience, assured Americans they were safe and, notably, toned down his typically full-throated call for congressional action on gun control.

After a briefing from his national security team on Thursday, Obama asked the American people and "legislatures" to find a way to make it "a little harder" for people to get guns.

"And we're going to have to, I think, search ourselves as a society to make sure that we can take basic steps that would make it harder - not impossible, but harder - for individuals to get access to weapons," he said.

The White House's reaction was viewed by some as an attempt to downplay the possibility that a terrorist attack had occurred on the president's watch and to divert attention.

"An act of evil unfolded in California. President Obama used it not as a moment to inform or calm the American people; rather, he exploited it to push his gun control agenda," National Rifle Association executive director Chris Cox said in a statement. "The plain truth is that the president cannot keep us safe. And his policies would leave us defenseless."

Republican presidential candidate Donald Trump also suggested Obama was dodging the issue.

"Our president doesn't want to use the term," Trump said Thursday. "But it turns out it probably was related - radical Islamic terrorism."

Republicans also have had to adjust their message. In the hours after the shooting, Trump initially cast such shootings "a mental-health issue, to a large extent."

The back-and-forth between the parties over gun control, gun rights, and the role of mental health in mass violence has become routine - in part because Obama, Clinton, and other prominent Democrats have begun to welcome the conversation.

In October, after nine people were killed by a gunman at Umpqua Community College in Roseburg, Ore., Obama said he wasn't afraid of politicizing the debate and vowed to keep up a steady drumbeat for new gun laws.