Washington (CNN)The United States is "certainly vulnerable" to becoming a new front line in the fight against ISIS, Sen. Ron Johnson said Sunday.
GOP senator: U.S. 'certainly vulnerable' to ISIS
Hear from the people who made the journey inside the minds and the hearts of ISIS. Watch Fareed Zakaria's special report, "Blindsided: How ISIS Shook the World," Monday at 9 p.m. ET/PT on CNN.
The Wisconsin Republican who chairs the Senate Homeland Security and Governmental Affairs Committee told CNN's Jim Sciutto on "State of the Union" Sunday that attacks inspired by ISIS, like one against a provocative cartoon contest in Texas a week ago, are allowing the group to convey a "winner's message."
"The best strategy the U.S. can employ to defeat this is actually defeat ISIS in Iraq and Syria so that the reality is conveyed that this is not a winning organization, it is a losing organization," he said.
Johnson admitted, though, that tracking ISIS sympathizers in the United States is a particular challenge, since the group has communicated with potential recruits over social media and because law enforcement officers can't track every possible suspect.
"The problem is, what do you do with the not-guilty-yet?" Johnson said. "We do have laws, we have a Constitution, and it's extremely difficult for law enforcement officials when you might have tens of thousands of sympathizers -- how do you track them all?"
Johnson cited a figure published in March: There are about 46,000 -- though maybe as many as 90,000 -- Twitter accounts that support ISIS.
"Now, Twitter is starting to shut those things down," Johnson said. "But just consider maybe 90,000 people drawn to this barbaric ideology. So we have got a very large haystack. We're looking for a needle in it."
Brett McGurk, the U.S. envoy for the Global Coalition to Counter ISIL, said 22,000 foreigners have joined the terror group's ranks in Iraq and Syria.
Of those, McGurk said, 3,700 are from Western nations, and 180 Americans have sought to join the organization. Fifteen of those Americans have been charged with supporting ISIS by the Justice Department.
"Long-term we are going to degrade and defeat this organization, but we have been clear from day one it's going to take a long time," McGurk said.
Two other experts said the threat of ISIS now is greater than what al Qaeda posed to the United States at the time of the 9/11 attack.
"We're in much more serious circumstances today than we were after 9/11," said Tom Ridge, who served as secretary of Homeland Security under President George W. Bush.
He said al Qaeda is "now a global scourge," and ISIS is growing, making the threat the groups pose "far more complicated today."
"Remember back then we thought about al Qaeda in Afghanistan and Pakistan and a few other places?" Ridge said. "Well, we've seen al Qaeda metastasize. It is now a global scourge. And you have the ascendancy of ISIL. The combination of those two groups -- their appeal to the lone wolfs and we see them acting in Belgium and in France and in Canada and the United States so the threat factors and the nature of the threats are far more complicated and far more serious today than on September 12, 2001."
Ridge proposed having local and state law enforcement officers help the FBI track ISIS sympathizers in the United States.
Mike Rogers, the Michigan Republican and former House Intelligence Committee chairman, said the FBI can't do any more to track ISIS sympathizers in the United States.
"If you treat this as a law enforcement issue that you're going to deal with, we are going to lose and we're going to lose badly," he said.
Homeland Security Secretary Jeh Johnson said on ABC's "This Week" that he's asking community leaders in meetings what they're doing to counter the narrative of ISIS -- which he said "is slick. It is effective."
Pushing back against that message, Johnson said, "has to come from within the community."
"It has to come from Islamic leaders, who frankly can talk the language better than the federal government can," Johnson said.
"And so when I meet with community leaders, Islamic leaders, that's one of the things that we urge them to do," he said. "Some have begun it. We've seen some good progress, but there is a lot more that can be done."