Professor Grant Reeher says people should be "wary" of attempting to identify a historical trend from just two cases: former President Bill Clinton and former President Donald Trump. However, he says the fact that three of four impeachment trials have happened in just over two decades "is in part a reflection of the deep political polarization the nation has been experiencing."

"Regarding impeachment, polarization has led the two parties to dig in immediately and deeply. Clinton's impeachment is probably a better pure example of its effects," Reeher says. "Trump's behavior has been far more problematic and dangerous to the nation, but I still think that polarization has affected how each party has responded, and it has made impeachment—or defending a president or former president against it—a more likely response," he adds. Read more in the Newsweek article, "Impeachment Trials Were Rare in U.S. History, Now Senate Begins Third in Just Over 20 Years."