Yes, indeed, the president’s primary mandate is to protect the nation against foreign and domestic harm. That’s it. So when a president places his own interest above that mandate, he makes himself liable to impeachment, crime or no crime.
Therefore, in this case, by President Trump advancing his own poltical interest and those geopolitical interests of the nation’s enemies above America and her allies is just about the worst form of leadership endeavor possible.
Criminal law is usually very specifically drawn up. Impeachment is more generally focused in order to protect all aspects of the nation’s security needs. The president’s action’s in regard to the president of Ukraine lies dead center of the impeachment intent to protect the nation from a dangerous rogue leader.