This means that they know that Rubio is a real threat to their stranglehold on power. It means that Rubio can win and destroy all that democrats have worked, lied and cheated for.
The left collects information on anyone who can or will become a force in the Republican party. When they need to destroy him, they bring out the information. If they don’t have any, they encourage someone to “remember” what the evil Republican did 30 years ago.
It also means that if Romney had picked Rubio to be his running mate instead of Paul Ryan, then Marco Rubio would be Vice President right now.