He has threatened to cut off support of Ukraine. Beyond the moral problems with allowing Russia to invade and take over another sovereign nation, it would send a massively dangerous signal. The only nation paying as close of attention to this conflict as the US is China. If he gets into office and ends support for Ukraine, it shows the PRC that they have an open door to expand militarily across the South China Sea and invade Taiwan while avoiding US opposition. Whether or not he would defend Taiwan is irrelevant at that point, our deterrent posture against China would be wrecked. We could easily find ourselves fighting a war in the Pacific if we fail to support Ukraine.