Do you really need this explained to you?
Ok, just once, for the hard of thinking:
President Obama was making a policy decision based on his interpretation of the situation. Nowi didn't agree with his policy decision, and neither did many folks in congress, but it was a policy decision. It's entirely legal and Constitutional to have policy disagreements.
dOnald tRump did not withhold the aid for policy reasons. He was trying to extort favors from the Ukrainian government. Nothing to do with policy. It is entirely illegal and unconstitutional to use the power of the office for personal gain.
Got it now?