Why do political parties in Congress sometimes fight, even when they agree? Is it like siblings who seem to quarrel over nothing — just the nature of the beast?

Frances Lee, a political scientist at the University of Maryland, agrees that a lot of the inter-party fighting seems senseless because it doesn’t involve deep philosophical differences. In her book, “Beyond Ideology: Politics, Principles and Partisanship in the U.S. Senate,” Lee writes, “The public perceives party conflict in Congress as ‘bickering,’ as excessive quarreling driven by members’ power and electoral interests.”

Political scientists, on the other hand, have “tended to interpret congressional party conflicts as evidence of members’ principled differences on the proper role and scope of government,” she writes.

Lee sides more with public perceptions that parties often spar just to advance narrow partisan interests, rather than giving voice to pre-existing policy differences in the larger political context. That only exacerbates and institutionalizes conflict. In their quest to win elections and hold power, she writes, “partisans impeach one another’s motives, question one another’s ethics and competence and engage in reflexive partisanship … rather than seeking common ground.”

Evidence of this can be found in instances in which the parties are in broad agreement on an underlying bill yet still engage in partisan combat. Lee’s analysis of the Senate reveals that “procedural votes on issues not involving ideological questions are just as intensely partisan as substantive votes on some of the most ideologically controversial issues in American politics.”

From my experience, the House is much the same. An example arose last month over House consideration of the Water Resources Reform and Development Act. The bill would authorize 23 water projects — dams, levees, canals, harbors, dredging and environmental restoration programs — at a cost of $3.1 billion over the next five years. It also would establish a new, non-congressional earmark process for selecting future projects.

The bill had nearly four dozen bipartisan co-sponsors and was approved on a voice vote from the 70-member House Transportation and Infrastructure Committee. Dozens of business, labor and civic groups endorsed the bill, as did the White House and bipartisan House leadership. With such a strong tailwind, it is little wonder the measure sailed through the House, 417-3.

And yet, before the vote, the special rule for the bill encountered partisan resistance. The Rules Committee had allowed one hour of general debate and 24 amendments — divided equally between the parties. However, 98 amendments had been submitted to the Rules Committee. Ranking Democrat Louise M. Slaughter’s attempt in committee to substitute an open amendment process was defeated on a party-line vote, as were attempts to make in order three additional amendments.

When the rule was called up on the floor, Rep. Alcee L. Hastings, who was managing the rule for committee Democrats, complained that the procedure blocked more than 70 amendments, “many of which were germane” — he said that “is not conducive to an open process.” The Florida Democrat then spent the balance of his time discussing something closer to home: Port Everglades, Fla., has been waiting 17 years for a report from the chief engineer of the Army Corps of Engineers on deepening its channels in anticipation of the new Panama Canal standards.

Near the end of the hour of debate on the rule, Hastings indicated that if the previous question on the rule was defeated (the only opportunity for the minority to amend the rule), he would offer a motion to make in order an amendment by Rep. Corrine Brown, D-Fla., to authorize projects that received a final chief of engineer’s report within a year after enactment, thereby holding out hope for Port Everglades. Despite Hastings’ efforts, the previous question was adopted on a near party-line vote, with only two Democrats breaking ranks, and the rule was subsequently adopted with all but 48 Democrats opposing it.

This minor partisan dust-up on the rule didn’t affect the eventual overwhelming passage of the bill. Hastings didn’t follow through on his implied threat to force a vote on an open amendment process and instead confirmed former Speaker Thomas P. “Tip” O’Neill Jr.’s axiom that “all politics is local.”

Nevertheless, the two procedural votes on the rule will be part of CQ Roll Call’s session-end tally of “party unity” votes (party majorities on opposing sides), as well as of ideological spectrum rankings of members. In the 112th Congress, 197 party unity votes on special rules alone (not counting other procedural votes) constituted 17 percent of all party unity votes — a significant exception to any ideology connection