Throughout the impeachment process of President Donald Trump, rancor has erupted between Republicans and Democrats in both the House and the Senate over whether Trump’s actions merit being removed from office. Unlike the prior impeachments of Richard Nixon and Bill Clinton, however, Trump’s impeachment appears to be split along party lines.

When Clinton was impeached, Republicans joined with Democrats in rejecting two of the articles of impeachment sent to the Senate. When Trump was impeached, however, neither article of impeachment garnered a single vote from a Republican.

In Ezra Klein’s book “Why We’re Polarized,” Klein seeks to find the root of the United States' modern polarization. During an interview with Boston Public Radio on Friday, Klein said that while the country is experiencing a unique form of polarization, it is not the most polarized the nation has been. During the Civil War era, Klein said, polarization was so bad that the nation saw war as the only answer to resolve their differences.

“[The Civil War] was a moment where we were so polarized that the only way forward was actually war,” Klein said. “So, this is not that bad.”

According to Klein, what makes today’s polarization unique is that the 20th century saw a period of relative non-polarization. The flashpoint, he said, came when the Civil Rights Act was signed into law in 1964. The signing of the act ruptured the previous makeup of both parties as many southern Democrats left to join the Republican Party while some liberal Republicans sought refuge in the Democratic Party.

“In the 20th century, we were depolarized. We were unusually nonpolarized by party, not just for us, but for any political system that is split by party in the world,” Klein said. “After that ideological polarization happens, then we begin polarizing by demographics, by race, religion, geography. And so it’s that stacking of ideology, of demographics, all that stacking on top of party.”