Nathan Newman has a fascinating post analyzing increasing political partisanship and legislative deadlock, with a basis in a quantitative analysis (pdf file) of Congressional voting patterns by Kenneth Poole and Howard Rosenthal.
Check out the evolution in the graphic below, and read Newman’s post and the article.

Having said that, I disagree with Newman in only one area. He says:

So what has happened?
Simple– the Civil War is over. We used to have different parties in different regions, especially between the South and the North, so nominal majorities by one party did not necessarily translate into a governing majority for policy. Back in Reagan’s first two years of office, we essentially had a similar 50-50 situation, where the GOP essentially ran the House with a narrow margin, despite official Democratic majorities, because the Boll Weevil’s supported the Reagan agenda. And it switched narrowly back to control by Tip O’Neill and the Dem leadership by a small margin in 1982. So we’ve had 50-50 margins of ideological control of Congress for decades now.
Today, the difference is that ideology is dressed clearly in party dress. While some Democrats crossed over to support Bush on final passage of his tax cuts, votes on all the amendments leading up to passage were tightly partisan, as I noted in Reflections on a Partisan Year. Aside from Miller, an extraordinary number of votes in the Senate fall along strictly partisan lines, something never seen earlier last century.

I see the roots of the change as being more mechanical than that. First, in the ability of party tacticans to manage redistricting, using better demographic data and computer analysis has led to the control of election results through voter selection through redistricting and gerrymandering. This is done by the parties, whose technicians ultimately control the process. Next, in the increasing cost of campaigning which must be borne by an aggressive fundraising structure…which structures are typically controlled by the parties, or by a cluster of consultants who rely on a steady stream of work from the parties for survival.
The administrator/bureaucrats in the parties are in control of the voters and the money; that would make it unproductive for a typical candidate to get crossways with them.