The "congress", or rather the military and civilian executives (Congress hasn't really weighed in yet) don't want to be in a position of having two large nuclear forces in direct conflict. For very good reasons.
Ukraine had nuclear weapons. Ukraine has weapon grade nuclear reactors, which produces plutonium. Ukrainian engineers developed equipment and rockets to produce nuclear weapons for USSR. Tactics of using nuclear weapons was part of my education.
Ukraine does not have nuclear weapons. They inherited some when the USSR fell apart, but they returned them to Russia to be dismantled in return for guarantees from the west and Russia.
The Chinese did not have nuclear weapons when the US fought them in the Korean War. Neither the Chinese nor the Soviets were (officially) involved in the Vietnam War. The US and Russia have danced around very carefully in Syria; the only direct conflict I know of involved Russian "mercenaries". In none of those cases did the Soviets or the Russians signal that they would use nuclear weapons; likewise, the Soviet invasion of Afghanistan was given covert military support by the US, but there was no direct conflict.
NATO operations in Ukraine carry a very large risk of accidental escalation, in addition to Putin's threats.
Ukraine had no nuclear weapons 8 years ago. Ukraine has all required equipment and nuclear materials (weapon grade Uranium, Plutonium) to create nuclear weapons at will. Are you sure that Ukraine has no will to defend itself?
> The US and Russia have danced around very carefully in Syria; the only direct conflict I know of involved Russian "mercenaries".
Turkey shot Russian airplane.
> NATO operations in Ukraine carry a very large risk of accidental escalation, in addition to Putin's threats.
I'm sorry to inform you, but war between two nuclear nations has much larger risk of escalation when NATO will play the role of coward.
According to what I've read and watched on YouTube (such as the interesting, if inevitably biased series "Russian Roulette" by ViceNews that I recommend you watch with your brain on and Wikipedia ready to look up every mentioned character), the Ukrainian army had such difficulties with basic supplies that I can't imagine them developing actionable weaponry at all, conventional or nuclear.
Even if some scientists could technically build a bomb, the hard part with nuclear arsenal is not the bomb itself but the launching device.
Nothing nuclear happened during the war with soviets in Vietnam, Korea, etc., or with Russians in Syria. RF is much weaker than USSR.