The American political process is inherently messy and disputatious. When I first came to Washington, in 1969, it was a period of divided government: A new Republican president, Richard Nixon, was faced with a Democratic Congress, marking the 15th consecutive year of their majority. Divisions were intense, particularly over the Vietnam War. But the divisions were not mainly along partisan lines. Among the strongest supporters of the Nixon approach to Vietnam were the Southern conservative Democrats (called "Boll Weevils" in honor of the insect that infects Southern cotton) who then made up about 40 percent of the majority in Congress, and also chaired most of the foreign policy and defense-related committees and subcommittees. Among the strongest opponents of Nixon's Vietnam policy were the northern liberal and moderate Republicans (called "Gypsy Moths" for the bug that hits Northeastern hardwood trees) who made up about a quarter of the minority party in Congress -- senators like Mark Hatfield of Oregon, who coauthored, with liberal Democrat George McGovern of South Dakota, the main legislative vehicle to pull the United States out of Vietnam.
Although Nixon's presidency ended prematurely in his second term under threat of bipartisan impeachment, he enjoyed a long period of productive domestic policymaking, with Democrats joining him and his party for legislative programs like revenue sharing and education reform. Sometimes Nixon was able to work with liberals, but more often he forged a centrist "conservative coalition" that included Southern Democrats joining most of his congressional Republicans.
Both Congress and the rest of the U.S. political system were already undergoing a sea change back then, with a realignment underway that turned the solid Democratic South into a more reliably Republican region and the strongly Republican New England and West Coast into Democratic strongholds. With those changes, the strong contingents of Boll Weevils and Gypsy Moths all but disappeared. Over time, the Democratic Party became more homogeneous and moved left, and the Republican Party became much more homogeneous and moved right. By the late 2000s, the center in Congress had virtually disappeared.
At the same time, two other phenomena emerged. The first was the permanent campaign, a change from an era where there were distinct seasons of campaigning and governing -- the former a vicious, zero-sum contest in which partisans were bitter enemies, the latter a conciliatory process of coalition-building among allies and adversaries alike. The second, precipitated by the stunning Gingrich-led Republican congressional victories of 1994, led to an extended era of close partisan margins, which gave rise to high-stakes legislative politics and sharply reduced incentives for lawmakers to work across party lines to solve problems.
If you doubt that politics in Congress has become more partisan, consider this: For the first time ever, in the 111th Congress that convened during the first two years of the Obama presidency, the National Journal's vote ratings showed that the most conservative Democratic senator was to the left of the most liberal Republican. There is now no overlap ideologically at all between the parties. Only nine of the remaining small number of conservative House Democrats (now called "Blue Dogs") were to the right of the most liberal House Republican. That Republican, Mike Castle of Delaware, was dumped by his party in a primary as he ran for the Senate and is now out of Congress, as are the bulk of the Blue Dogs.