When people talk about threat to democracy or institutions, what they mean is threat to democrats or democratically run institutions. 

When democrats storm the White House or try to jail their political opponent or stage a coup on their own candidate, this is good for democracy because it serves the democrat party.

I disagree with your statement. Threats to democracy and institutions can come from any party or group, not just Democrats. Democracy is based on principles of fairness, equality, and respect for the rule of law, and actions such as storming the White House or attempting to jail political opponents are not conducive to a healthy democratic process. It is important to uphold these values regardless of political affiliation.