WASHINGTON, DC—The attorney for a federal judge facing removal from office argued Tuesday at a rare Senate impeachment trial that Congress is pursuing vague and unconstitutional charges against his client and would be breaking with precedent by convicting him. Defense attorney Jonathan Turley told senators assembled in the chamber for the historic proceeding that some of the articles facing Judge G. Thomas Porteous are subjective and that others involve conduct that occurred before Judge Porteous was appointed to the federal bench. "In the history of this republic, no one has ever been removed from office on the basis of prefederal conduct," Mr. Turley said, urging the senators to dismiss some of the most serious allegations. Mr. Turley's arguments came as the Senate began the final stage of the case against Judge Porteous, a U.S. district-court judge from Louisiana. The House voted unanimously in March to bring four articles of impeachment against him. A two-thirds Senate vote is needed to convict. The proceeding is just the 16th impeachment trial before the Senate, and Judge Porteous could become just the eighth federal judge to be removed from office. House prosecutors allege that Judge Porteous was racking up debt as he struggled with drinking and gambling problems. They say he began accepting cash, meals, trips and other favors from people with business before his court, beginning as a state judge in the 1980s and continuing after he was appointed to the federal bench by President Bill Clinton in 1994. During previous evidence-gathering hearings, two attorneys who once worked with Judge Porteous said they gave him thousands of dollars in cash, including about $2,000 stuffed in an envelope in 1999, just before he decided a major civil case in their client's favor. They also said they paid for meals, trips and part of a bachelor party for one of Judge Porteous's sons in Las Vegas. One of the attorneys said they paid the judge in cash to avoid a paper trail and that some of the transactions probably amounted to illegal kickbacks. Another said Judge Porteous's requests grew so common that he started trying to dodge the judge. Another witness, New Orleans bail bondsman Louis Marcotte, described a longstanding relationship in which Mr. Marcotte and his employees routinely took Judge Porteous to French Quarter restaurants, repaired his automobiles, washed and filled his cars with gas, and took him on trips. In return, Judge Porteous manipulated bond amounts for defendants to give Mr. Marcotte the highest fees possible, said Mr. Marcotte, who served 18 months in prison on related corruption charges. Judge Porteous, who sat with his attorneys Tuesday before the chamber, also stands accused of filing for bankruptcy under a false name and lying to the Senate during his judicial confirmation. Mr. Turley has argued that Judge Porteous may have made poor decisions but that his actions don't rise to the "high crimes and misdemeanor" standard required by the Constitution for impeachment. He has sought to portray much of the judge's behavior as business as usual in the New Orleans-area legal community. The Senate is planning to vote on the case Wednesday.