We need to put partisan interests aside to defend Ukraine and defeat Putin.

I just returned from a weeklong congressional member delegation to Ukraine, Poland and Moldova. Ukraine faces a new Russian offensive with insufficient ammunition. My first social-media post reporting on our trip was greeted with a flood of hostility and propaganda, I suspect mostly generated by Russian bots, including a doctored photo of me wearing a Nazi armband.

On my return to Washington, I was greeted by Rep. Marjorie Taylor Greene’s newest threats to remove Speaker Mike Johnson if he brings to the floor a bill to aid Ukraine. Democrats and Republicans must join together to save Ukraine and stop the disinformation campaigns promulgated by Vladimir Putin and furthered by Mrs. Greene.

The Russians have killed more than 100,000 Ukrainians, kidnapped more than 20,000 children, and chased more than 10 million Ukrainians from their homes. Mr. Putin’s goal is to push Russia’s borders farther west, to control Ukraine’s substantial agricultural exports and natural resources, and to destabilize Europe’s and the world’s democracies. In addition to Russia’s substantial military might, Mr. Putin and our other strategic adversaries have employed a decades-long disinformation campaign using social media and state-controlled media to get Americans fighting among ourselves.

According to leading Republicans, Mrs. Greene is parroting Russia’s propaganda. House Intelligence Chairman Mike Turner said this week: “We see directly coming from Russia attempts to mask communications that are anti-Ukraine and pro-Russia messages—some of which we even hear being uttered on the House floor.” Foreign Affairs Chairman Michael McCaul said this propaganda has “infected a good chunk of my party’s base.”

“The ignorance of one voter in a democracy impairs the security of all,” John F. Kennedy said. What happens if Russia convinces Americans of its lies? Kyiv is running out of the antiaircraft weapons it uses to defend from bomb-carrying drones. When will we begin running interference on the misinformation online that is a direct threat to our democracy?

Mr. Putin didn’t invent American partisanship, but his lies and propaganda have fed it, giving rise to extremists like Mrs. Greene, who would rather America do nothing with democracy on the line. This fight isn’t only about Ukraine. Russia seeks to tear apart the fragile fabric of international order, end American prominence in Europe, destroy the North Atlantic Treaty Organization, and end the era of American-style democracy. America must stop Mr. Putin before our men and women in uniform are forced to defend NATO soil.

Republicans in the House must listen to their intelligence chairman and follow the lead of the Senate, which passed a $95.3 billion foreign-aid bill in a bipartisan 70-29 vote. Meanwhile, Democrats can’t just wait for Republicans to work with us. That’s why when Mrs. Greene introduced her proposal to remove Speaker Johnson, I announced that I will support him if he does his job.

Democrats must offer Speaker Johnson our votes to save democracy in Ukraine and here. We can’t let our partisan instincts get in our way. We must work with Republicans to disarm Mr. Putin’s puppet, get a vote to support Ukraine, and defeat Russian disinformation. Our democracy is at stake.

Mr. Suozzi, a Democrat, represents New York’s Third Congressional District.