Let's go with the instances I cited. They are the ones with the largest consequences, and let's not get too fancy.
Truman firing MacArthur. If he had his way the US would have nuked China, and been nuked in return. Because of a war on the Korean peninsula. A conservative president would probably given that war hero his head, Truman had the guts to fire him.

The Cuban Missile crisis. The consensus of his advisors was to attack Cuba. Had Nixon been in office, with his "hard on communism" reputation he would have taken the advice of his folks. Result would have been millions upon millions of people dead. As it was, Kennedy got the missiles out at a minimal cost. Thank God.

The economy inherited by Clinton was tanking fast. I remember that. Clent left Bush the Second a trillion dollar surplus. Buth squandered it, and left Obama with a seriously tanking mess, so bad it even scared Bush. And Obama is leaving an economy which seems to be in good shape.

Now one can get fancy, and obfuscate, drag in things not mentioned, and confuse matters in the finest tradition of conservatives on the HZ. I'm related instances I remember in plain, simple English.

Thank God for Democrats protecting us from Conservative ineptitude. "If your child can speak at all, thank a Democrat." "If your child doesn't glow in the dark thank a Democrat." "If YOU can read this, thank a Democrat!"

Or if you like broke, homeless, radiation sickness and no health care, vote Republican. It really is that simple.