Imagine waking up Kamala Harris, "uhm… morning Ma'am, you're POTUS now, are we declaring war on Russia?"
Had Biden been killed, Russia would have denied any involvement and blamed Ukrainian "terrorists". Unless Harris has definitive proof that Russia made the hit, I don't think a deeply divided US Congress would agree to declare war on a nuclear armed Russia. And what would the US war aims be? You killed our president, so we get to kill yours - and for that we're willing to risk millions of American lives?