Asymmetric polarization, as espoused by analysts like the Brookings Institute's Thomas Mann, goes something like this: Democrats have created a big tent governing majority that tolerates lots of different views and, as a result, has a kind of "moderate center" that has remained relatively consistent; while Republicans have exiled the centrists in their party as grassroots voters have taken over and made their party more extreme all on their own.
That's flatly not true.
The typical measurement of asymmetric polarization comes from a Congressional metric called DW-NOMINATE, which at its heart measures how often elected representatives in Congress vote with each other. The flaw here is that it doesn't take into account the relative leftness or rightness of a proposal being voted on. If there were a vote in Congress to repeal the 2nd Amendment, for example, with all Republicans voting against and Democrats split evenly between for and against, would be a data point contributing to Republicans becoming more "extreme" and Democrats being relatively centrist. DW-NOMINATE has certain utility in evaluating legislative coalitions, but as an actual measure of how left-wing or right-wing any coalition is, it's severely lacking.
Pew's survey finds a lot of interesting trends in the Republican Party, but one thing is certain: Democrats have gotten much more liberal over the last twenty years. Pew actually measures how people feel about a lot of different issues, not just how their votes cluster. Democrats moved noticably leftward in the late 90s. What is true is that Democrats stagnated throughout the Bush years in how Pew measures how liberal they are - they remained relatively consistent. But in the Obama era, Democrats have veered sharply leftward, as shown in these two images; watch how far the median Democrat jumps from 2010 to 2014:
Congressional Democrats will probably move farther to the left next year. Molly Ball writes at The Atlantic:
This year’s Republican primaries have been closely watched by pundits sifting for clues about the relative primacy of the GOP’s warring factions. But Democrats have primaries too—and this year, the left is winning many of them.
National liberals point to a handful of recent contested primaries where candidates from the party’s “Elizabeth Warren wing” beat moderate “corporate Democrats” to argue that the left wing is on the rise. It's a similar dynamic to the Tea Party-vs.-establishment divide on the right, though far less divisive, and a trend that has the potential to quietly reshape the Democratic Party if it continues.
In New Jersey, Bonnie Watson Coleman, a former assemblywoman who campaigned on raising taxes on millionaires to spend more on education, had been thought to be in a close race with a moderate state senator, Linda Greenstein. Instead, Coleman won the primary by a double-digit margin.
In Iowa, Pat Murphy, a former state representative, aired TV ads that dubbed him a “bold progressive.” He beat out four opponents, taking 37 percent to his nearest competitor's 24 percent.
Victories like these have led the Progressive Change Campaign Committee to declare vindication for its view that Democrats win when they campaign on a platform of muscular liberalism. “A message of economic populism is what actually excites voters and drives them to the polls,” Adam Green, PCCC’s co-founder, told me. “Democrats are shooting themselves in the foot if they don’t embrace it.”