Until the election of Donald Trump, no sitting president had ever requested a foreign government’s help to discredit a political rival. Coupled with Trump’s appeal to Russia during the 2016 campaign that Moscow use its cyber power to uncover Hillary Clinton’s missing emails, not to mention his eldest son’s eagerness to accept anti-Clinton material from Kremlin allies, Trump’s willingness to allow foreign governments to influence American elections is historically unprecedented.

Just how unprecedented becomes clear when you look back at the long history of attempts by foreign powers (almost always Russia) to tip an outcome to their advantage. On multiple occasions since the start of the Cold War, Moscow has proffered money, dirt and manpower to undermine a candidate perceived to be harmful to their interests. But in nearly every instance, the interference never came to pass. And this is the starkest difference between Trump and other presidential candidates—and between Trump and every one of his presidential predecessors. Where Trump has welcomed such assistance—and, in the case of his controversial call to Ukrainian president Volodymyr Zelensky, demanded it—other candidates, to a man, rejected the aid.

What these examples show is that across parties and across decades—whether or not there were laws in place banning the foreign aid—aspirants to the nation’s highest office recognized the impropriety of the offers. Even as they knew how valuable it might be to them, especially as challengers, they understood that accepting the assistance would compromise them and the underpinnings of American democracy, should they win.

“I can’t think of any precedent for this kind of prima facie corrupt action on the part of an American president,” Brad Simpson, an associate history professor at the University of Connecticut with a focus on U.S. foreign policy, said. “I think that [this is] a president whose whole political life has been prone to conspiracy theories, but who now has the apparatus of the executive branch to try and do something about it—and that’s what’s really novel.”