Let's assume no Constitution amendments are allowed. Constitution: A man may only be elected for the U.S. presidency for 2 terms or serving 10 total years max. Yes, Barack Obama can become U.S. President again under the 1/millions of chance. If a very old politician like Bernie Sanders or Joe Biden runs as the democratic presidential nominee and Barack Obama joins as a vice presidential candidate. Assume they get elected in 2020. Constitution: The vice president will automatically become the next president if the current president dies in the office. Barack Obama is 56 years old and has served 8 years, he will be at age 60 by Aug, 2021. Well, 10 total years max means 2 more legitimate years for Obama under this Scenario and he can pick a new vice president to succeed him like how Gerald Ford did in the 1970s. So it's possible for 3 democrats to become U.S. presidents within 4 years.