The president even raised the prospect that an emboldened Mr. Putin would pose a threat to NATO allies, requiring the United States to come to their assistance with troops on the group. “If Putin takes Ukraine, he won’t stop there,” Mr. Biden said. “It’s important to see the long run here. He’s going to keep going. He’s made their pretty clear.”

“If he keeps going and then he attacks a NATO ally” to which the United States is bound by treaty to help, “then we’ll have something that we don’t seek and that we don’t have today — American troops fighting Russian troops,” Mr. Biden said.

“Make no mistake,” he added. “Today’s vote is going to be long remembered and history’s going to judge harshly those who turn their back on freedom’s cause. We can’t let Putin win. I’ll say it again, we can’t let Putin win.”