During an interview on Friday, Ron Reagan said his father would never support Mr Trump and blasted him as “a traitorous president who is betraying his country”.
He also criticised the current Republican Party which he described as “entirely illegitimate” and “made up of a bunch of sycophantic traitors mouthing Kremlin propaganda to defend this squalid little man who is occupying the White House”.
“This is a dying party. They either have to remake themselves entirely or they will disappear eventually. Within a decade the Republican Party will be a minor fringe group if it continues going this way,” Mr Reagan told The Daily Beast.
“My father would have been ashamed of this Republican Party. He would have been embarrassed and ashamed that a president of the United States was as incompetent and traitorous as the man occupying the White House now. He’s a disgrace to the office of the presidency.”
The former president’s son also likened the current political turmoil in the US to the run-up to the Civil War, predicting Mr Trump will attempt to unconstitutionally seek a third or fourth term.
He claimed Mr Trump “wants to be president for life” because “it would keep him out of jail”.
Mr Reagan continued: “It is perhaps the most dangerous time for our republic in my lifetime, perhaps since the Civil War. If there is a second Trump administration, he’s going to feel liberated to do whatever he wants to do.
“He’ll try and go for the third, fourth, however long he lasts. He wants to be president for life. It would keep him out of jail, for one thing.”
Mr Reagan describes himself as an “unabashed atheist” who is “not afraid of burning in hell”. He also greatly differs from his father - a Republican of almost four decades standing - in terms of his political views. However, he still maintains his father “held the office with a degree of dignity and class”.
When discussing Mr Trump’s recent cosying-up to evangelical Christians, Mr Reagan dismissed the move as “cynical” and political.
He concluded: “They support him, he’ll give them whatever they want. How funny he looks when he closes his eyes to pray, or just looks sullen when he doesn’t want to sing hymns. It’s entirely cynical. This is about his political future.”