The Bush White House asserted its executive prerogative on multiple domestic and national security issues, and the Republican-led Congress, in the aftermath of the Sept. 11, 2001, attacks, showed little interest in holding the administration accountable, despite widespread criticism.

“You have tended to get a complete abdication of oversight during periods of Republican unified government,” said Thomas E. Mann, a longtime congressional scholar affiliated with the Brookings Institution.

It has not always been this way. In the not-too-distant past, lawmakers viewed themselves as more independent, separate and even superior to administrations that came and went while members of Congress built long careers on Capitol Hill.

They were more than willing to take on a president of their own party as Democrats did over the conduct of the Vietnam War. And Republicans were full participants in bipartisan inquiries into Watergate and, during the Reagan era, the Iran-contra affair. The readiness of the parties to cooperate in serious joint investigations gave them much more credibility as opposed to the partisan recriminations surrounding the now two-year-old House Benghazi investigation.

Much of the most serious scrutiny of any administration came during the annual appropriations reviews, when members of both parties held the feet of agency officials to the fire.

But that process is badly broken now. And the increasing polarization of American politics has led to a partisan “we’re all in this together” mentality where members of Congress have been very reluctant to show any separation between them and their party’s leader for fear of providing the slightest political opening to the opposition.

For instance, Republicans with reservations about conduct during the invasion of Iraq or the response to Hurricane Katrina did not want to give ammunition to Democrats by airing their grievances despite very real misgivings. And Democrats have been careful about criticizing Mr. Obama publicly even when they sometimes found the administration’s actions amateurish and counterproductive.