Could be the 20 Trillion in debt ?
Maybe allowing North Korea to develop and build ICBMs ?
Possibly allowing and funding , in cash , Iran's pursuit of nuclear weapons capability ?
Some might say the historic social and racial discord he created
Might even be the sabotage and vandalism to the medical insurance industry
Myself ,,,, I say it was his weaponizing of federal agencies , The IRS , The EPA , The DOJ , The FBI and CIA against American citizens and his political rivals .
0bama did a great deal of long term , in some cases permanent damage .