More than 400,000 people have been killed in the Syrian civil war, according to the United Nations.

But Mr. Johnson complained that presidential candidates were expected to talk tough about dealing with dictators like Mr. Assad or risk losing support.

“This is what happens in this country right now — unless you’re willing to say that you’re going to get tough on this stuff, on these atrocities — and these atrocities are horrible — but unless you as a politician are willing to do something about these atrocities then we’re not going to elect you,” Mr. Johnson said.

Mr. Johnson speculated about the deals or promises that the Obama administration may have dangled before anti-Assad forces, but then said at other points that he was at a disadvantage discussing world events because he has not received the classified briefings that the Democratic and Republican nominees for president have been given.

Mr. Johnson also declined to say whether his running mate, former Gov. William F. Weld of Massachusetts, had warned him of his intention to change course and spend the remainder of the presidential campaign attacking Donald J. Trump, the Republican nominee, as unqualified. Mr. Weld revealed those plans in an interview with The Boston Globe on Tuesday.

“That may be his primary mission,” Mr. Johnson said of Mr. Weld’s plans. “We’re not scripted at all. And so I guess my role will be Hillary and his role will be Donald Trump.”

But he insisted that Mr. Weld would not drop out at any point and endorse Mrs. Clinton. “Absolutely,” Mr. Johnson said when asked if Mr. Weld would fight on until Nov. 8.

Mr. Johnson said that the idea of hunkering down and trying to make an aggressive play to win his home state of New Mexico, where polls show him in contention, had crossed his mind. But he conceded that his chances of winning outright were all but gone.

“Right now it’s a Hail Mary, not being in the presidential debates,” he said.