Voting to send Ukraine all of the weapons and resources they can possibly use. That is the only legitimate anti-war stance. It's a remarkably black and white scenario where an authoritarian country invaded its democratic neighbor. Russia could end the war immediately simply by leaving. Ukraine, on the other hand, needs to actually win the war. Anything other than an absolute Ukrainian victory where Russia is forced out of all of Ukraine is a victory for war. The real-world anti-war position is that war must not be allowed to be an effective means of aggression. If war works, we get more war. When dictators learn that war doesn't work, we'll have less war. As long as Putin isn't willing to pull back his soldiers and abandon all claims to all Ukrainian land, the only other option is to defeat him in battle.
Anybody who opposes arming Ukraine is objectively pro-war because the only plausible outcome in that scenario is a Russian victory.