Attorney General William Barr said the Obama administration should have asked the 2016 Trump campaign about possible collusion with Russia, saying the investigation the FBI pursued instead was based on a "flimsy" premise.

"Usually, we don't assume, as the government, that the campaign is part of a foreign plot," Barr said at the WSJ CEO Council meeting on Tuesday. "The idea that you only talk to the Americans if you are 100% sure that they are not culpable is not true. I don't think there is a legitimate explanation for why they didn't go to the campaign."

"It is a big deal to investigate a campaign. I can't think of a recent instance when the government investigated a political campaign," he said.

"If your purpose was to protect the election, you would have done a defensive briefing," Barr added. "The chances of [Jeff] Sessions or Chris Christie being in cahoots with the Russians was pretty minimal."

Barr alleged that "to pore over U.S. code to try to find some esoteric argument" to pursue a target was akin to using criminal law as a weapon. "And then they start calling for scalps," he added.

He noted that, on Aug. 4, 2016, officials "contacted Russian Intelligence and said, 'We know what you are up to — you better stop it.'" It didn't make sense that the administration would talk to the Russians but not the campaign, he said.

"We're not going to talk to the campaign; we're going to wire people up and send them into the campaign,'" Barr said. "And it came back exculpatory."

Barr described a remark George Papadopoulos, a onetime Trump campaign aide, made in a bar as "off-hand" — "a suggestion of a suggestion" — and a "flimsy" excuse to launch an investigation into the campaign. He said speculation was already rife that Hillary Clinton's email server had been hacked "and that the Russians might have the server." That Trump "showed preknowledge," Barr said, "is a stretch."

"At that point, it was clear the dossier was a sham," he said. "It is hard to look at this stuff and not think it was a gross abuse."