There’s no question that, after almost eight years of President Obama, the Democratic Party is more liberal than it’s been in a generation. Today’s Democrats support same-sex marriage and marijuana legalization, favor universal health care, endorse criminal justice reform and Wall Street regulation, and want stronger action on climate change. Not only does Obama support these policies, but Hillary Clinton has launched her presidential campaign with related commitments to equal pay, family leave, comprehensive immigration reform, and an overhaul of the prison system. And facing her is pressure on her left from liberals such as Sens. Elizabeth Warren and Bernie Sanders who want to go further.

To Peter Wehner, a conservative writer and former Republican official, this is prime evidence of an ideological shift in the Democratic Party. “Among liberals,” he writes in the New York Times, “it’s almost universally assumed that of the two major parties, it’s the Republicans who have become more extreme over the years. That’s a self-flattering but false narrative.” Republicans are more conservative, he admits, but the reality is that “in the last two decades the Democratic Party has moved substantially further to the left than the Republican Party has shifted to the right.”

Some of this, says Wehner, is separate from any trend in the Democratic Party. “In some respects, like gay rights, the nation is more liberal than it was two decades ago.” But overall, he says, it’s a function of “Obama’s own ideological predilections and the coalition he built.” The upside is a liberal agenda. The downside, he argues, is political disaster. “For demographic reasons, many Democrats believe that they are riding a tide of presidential inevitability. They may want to rethink that.”

If the Democratic Party had veered sharply to the left, surpassing the GOP’s turn to the right, this would be convincing—a fair and timely warning to liberal activists and left-wing agitators. The problem for Wehner, however, is that this is a Bizarro take on the state of modern American politics.

Here, Thomas Mann of the Brookings Institute and Norman Ornstein of the American Enterprise Institute are helpful. “Since the late 1970s,” they write in It’s Even Worse Than It Looks: How the American Constitutional System Collided With the New Politics of Extremism, “Republicans have moved much more sharply in a conservative direction than did Democrats in a liberal direction.” You can see this most clearly in the ideological profile of the 112th Congress, elected in 2010. “Nearly 80 percent of the freshmen Republicans in the 112th Congress would have been in the right wing of the party in the 111th Congress.” ...