You do understand that Ukraine regularly hits Moscow and has even hit the Kremlin?

With drones. A balistic missile strike is different from a nuclear missile strike only in the warhead in the missile. Firing missiles that could potentially be nukes (from the Russian perspective) from a nucelar power (the USA) at their capital is one of the few things that might actually trigger a nuclear war. Not the idiotic Russia bluster about Ukraine being given access to slightly better weapons, but a genuine possibility.