1) The electorate is not "polarizing." It's "sorting."
An electorate is "polarized" if voters are increasingly drawn to extremes -- the right getting more conservative, the left getting more liberal, and moderates dwindling. An electorate is "sorted" if voters are increasingly settled into ideological camps, that is, conservatives are almost all Republicans, liberals almost all Democrats.
Pundits talk all the time about "polarization," but it's not happening. As Fiorina points out, the percentage of Americans who call themselves "moderate" is the same as it was in the 1970s (the American National Election Studies survey has put it at between 20 and 30 percent since 1972). Nor are we more divided when it comes to issues. In the words of a 2012 Pew study, "The way that the public thinks about poverty, opportunity, business, unions, religion, civic duty, foreign affairs, and many other subjects is, to a large extent, the same today as in 1987. The values that unified Americans 25 years ago remain areas of consensus today, while the values that evenly divide the nation remain split." The commonplace idea that Americans today are irrevocably divided into politically extreme camps just isn't the case.
What has happened to the American electorate in recent decades is sorting. A few decades ago, there were thriving factions of conservative Democrats and liberal Republicans, among both voters and their elected representatives; that is to say, the parties themselves were internally diverse. Nowadays, however, ideological consistency is the rule. This is the real reason behind many phenomena commonly, and incorrectly, attributed to "polarization," Fiorina points out, such as the massive decline in ticket-splitting. Today, voters are likely to find that all the candidates who agree with their views belong to the same party, whereas in the 1970s, many House and Senate candidates didn't have much in common with their party's presidential nominee. Sorting also accounts for voters' increased party loyalty. They haven't necessarily become more rigid -- they're just more likely to find all the candidates they support concentrated on one side of the aisle.
There's an important footnote to this discussion: While voters haven't become more polarized, Congress has. There are far fewer moderates in the House and Senate, and members have increasingly inclined toward the ideological extremes.
2. Candidates change more than voters do.
Commentators, Fiorina observes, often point to swings in voter preference as evidence that voters are changing. But voters are more often responding to their changing slate of choices. "Did Republicans defect more than usual in 1964 (Goldwater) and Democrats more than usual in 1972 (McGovern) because their party identifications experienced a sudden weakening in those years?" Fiorina asks. Not really -- what happened was that in each case, the candidate in question was too extreme to appeal to the broad spectrum of his own party. That's an obvious example, but it applies just as well to Bush-Kerry or Obama-Romney. Claims about the electorate "swinging" toward one ideology or another are likely to be overblown.