Thomas E. Mann is a senior fellow at the Brookings Institution, and Norman J. Ornstein is a resident scholar at the American Enterprise Institute. This essay is adapted from their book “It’s Even Worse Than It Looks: How the American Constitutional System Collided With the New Politics of Extremism,” which will be available Tuesday.

Rep. Allen West, a Florida Republican, was recently captured on video asserting that there are “78 to 81” Democrats in Congress who are members of the Communist Party. Of course, it’s not unusual for some renegade lawmaker from either side of the aisle to say something outrageous. What made West’s comment — right out of the McCarthyite playbook of the 1950s — so striking was the almost complete lack of condemnation from Republican congressional leaders or other major party figures, including the remaining presidential candidates.

It’s not that the GOP leadership agrees with West; it is that such extreme remarks and views are now taken for granted....

It is clear that the center of gravity in the Republican Party has shifted sharply to the right. Its once-legendary moderate and center-right legislators in the House and the Senate — think Bob Michel, Mickey Edwards, John Danforth, Chuck Hagel — are virtually extinct.

The post-McGovern Democratic Party, by contrast, while losing the bulk of its conservative Dixiecrat contingent in the decades after the civil rights revolution, has retained a more diverse base. Since the Clinton presidency, it has hewed to the center-left on issues from welfare reform to fiscal policy. While the Democrats may have moved from their 40-yard line to their 25, the Republicans have gone from their 40 to somewhere behind their goal post.

What happened? Of course, there were larger forces at work beyond the realignment of the South. They included the mobilization of social conservatives after the 1973Roe v. Wade decision, the anti-tax movement launched in 1978 by California’s Proposition 13, the rise of conservative talk radio after a congressional pay raise in 1989, and the emergence of Fox News and right-wing blogs. But the real move to the bedrock right starts with two names: Newt Gingrich and Grover Norquist....