Why are partisan differences so sharp and bitter in America today? It's a question many have been asking and trying to answer. On the Washington Post's Monkey Cage blog, University of Maryland political scientist Frances Lee gives an interesting explanation: It's because American politics these days -- which is to say, over the last 20-some years -- has become very competitive, more so than at most times in American history. “Competition for power,” writes Lee, “not only ideological polarization, contributes to our confrontational politics.”

Lee illustrates her proposition with a graphic showing an Index of Two-Party Competition in every decade from the 1860s to the still-far-from-complete 2010s. There are decades of substantial one-party dominance: the Civil War 1860s for the Republicans (when the mostly Democratic South usually wasn’t voting) and the Great Depression 1930s for the Democrats. Republicans also had significant dominance in the 1900s and 1920s. The five decades after the Depression — the 1940s, 1950s, 1960s, 1970s and 1980s — showed varying degrees of Democratic dominance.

The most recent three decades -- the 1990s, 2000s and 2010s -- showed record low levels of one-party dominance, less than any other decade against the 1880s. This matches my own observation that there was less variance in the two parties' percentages of the popular vote for the House of Representatives in the five elections between 1996 and 2004 than at any time since the 1880s. And, as I like to say, unfortunately no one in Washington today has any living memory of the 1880s now that Strom Thurmond is gone.

Lee argues that close two-party competition tends to produce partisan bitterness. At a time when elections are close and outcomes matter (think: Iraq war, Obamacare), officeholders, candidates and voters are going to be more bitter than at times when it is taken for granted that one party will win majorities year after year. Let me take a step beyond this and argue that bitterness tends to increase not because of the weaknesses and missteps of politicians but because of their competence as political competitors.

As the 1990s began, journalists and political scientists argued that Republicans had a lock on the presidency and Democrats had a lock on the House of Representatives. Structural arguments were made suggesting that this would always be so, and they seemed plausible at a time when Republicans had won five of the last six presidential elections, with an average popular vote margin of 10 percent, and Democrats had held majorities in the House for an unprecedented 36 years, since 1954. Yet within four years both locks were broken, and in both cases with leadership from skilled and insightful politicians of both parties -- Bill Clinton and Newt Gingrich.

In the two decades since then, Democrats have won four of six presidential elections (and won a plurality of the popular vote in a fifth) and Republicans have won majorities in the House in eight of the last 10 elections. And these results have occurred despite a marked increase in straight-ticket voting. Their margins, however, have been smaller than the other parties’ margins in the pre-1990 period. The average Democratic popular vote margin in presidential races has been 4 percent, not 10 percent. And Republicans from 1994 to 2012 never won as many seats in the House as Democrats did in all 18 elections between 1958 and 1992.

In the years since the Clinton and Gingrich breakthroughs both parties, I would argue, have tended to compete competently under circumstances in which there was considerable uncertainty about which one would prevail. In every presidential election since Clinton was re-elected in 1996, there has been uncertainty about the outcome (even in 2008, when John McCain led Barack Obama in polls during the first half of September; though some would argue the point). And in every House election cycle with the exception of 2008 and the possible exception of 2012, there has been considerable uncertainty whether the party in power would retain its majority.

So as you listen to those laments about that golden age when politicians of opposite parties got along with each other — a golden age that recedes farther in the distance as you seek it — keep in mind that partisan bitterness is the product of competent party competition. Partisans get along less because parties compete more ably. It’s proof again that you often can’t have two good things — political comity and political competition — at the same time.