WASHINGTON  With high-profile investigations under way against Democrats and Republicans, Congress is facing a series of difficult tests of the toughened ethics system that it put in place to weed out corruption and malfeasance among its members.

Two years ago, after a scandal that centered on the disgraced lobbyist Jack Abramoff, the House created an independent ethics office as part of what Speaker Nancy Pelosi called an effort to end the “culture of corruption” in Washington. The Senate also took action, setting up what it described as tough new regulations.

Since then, however, no member of Congress has been censured, the toughest punishment short of expulsion, despite a number of recent scandals involving sexual impropriety, financial dealings and conflicts of interest. The record illustrates how Congress has struggled to police itself after years in which its ethics committees were often derided as ineffectual.

For instance, two weeks after public disclosures raised questions about the involvement of Senator John Ensign, Republican of Nevada, in possible illegal lobbying, Senate ethics officials have yet to contact the former top Ensign aide at the heart of the case, even though they portray it as a serious matter. Meanwhile, the investigation of the finances and fund-raising of Representative Charles B. Rangel, Democrat of New York, has dragged on for more than a year and has become the subject of tense political infighting.