Richard Leon, the judge who declared that the National Security Agency's bulk collection of millions of Americans' telephone records is likely unconstitutional, has a long record of taking on executive branch actions.
Leon, a blunt-spoken, bowtie-wearing appointee of Republican President George W. Bush, has ruled against the federal agencies on issues from drugs used for executions to warnings on cigarette packages.
The 64-year-old judge writes with flair, occasionally using exclamation marks to amplify a point.
In a 2009 ruling ordering the release of a Guantanamo detainee, he wrote in response to a government argument, ''I disagree!''
In Monday's ruling, he wrote that ''there is the very real prospect that the (NSA) program will go on for as long as America is combatting terrorism, which realistically could be forever!''
His bluntness was on display at a hearing in the NSA case last month, before he decided how he would rule.
''I mean, I am not kidding myself,'' he told both parties, predicting the case would go to the Court of Appeals and probably the Supreme Court after that.
''It doesn't matter, however I rule.''
At a previous hearing, Leon admonished a government lawyer who noted that a deadline set by the judge fell on a federal holiday.
''We work 24/7 around this courthouse, my friend, 24/7,'' Leon said.
''I don't want to hear anything about vacations, weddings, days off. Forget about it.
This is a case at the pinnacle of public national interest, pinnacle. All hands 24/7. No excuses.''
He has decided several Guantanamo Bay cases, including one in 2008 in which he ordered the release of five detainees after concluding that the government's evidence linking the men to al-Qaida was not credible as it came from a single, unidentified source.
''To allow enemy combatancy to rest on so thin a reed would be inconsistent with this court's obligation,'' Leon said.
But in a ruling the following year, Leon said the government could continue to hold an assistant cook for Taliban fighters.
''After all, as Napoleon himself was fond of pointing out, an army marches on its stomach,'' Leon said in issuing the ruling.
Leon, a graduate of Harvard Law School, had previous stints including the Justice Department, Capitol Hill and private practice before joining the court in 2002.
Bobby Burchfield, a partner at McDermott Will & Emery who has known Leon since his days on Capitol Hill, said he wasn't surprised that Leon would be willing to issue a ruling that would have a big impact on the government.
''He comes from a prosecutorial background, so it isn't that he has it in for the government,'' Burchfield said in a telephone interview.
''Knowing him, I give him the benefit of the doubt that he looked at it carefully and found something that bothered him.''
Last year, Leon blocked the importation of a drug used in executions, ruling that the Food and Drug Administration ignored the law in allowing it into the U.S.
''Few in our society are more vulnerable than a death row inmate facing lethal injection,'' he wrote.
In another ruling, he blocked a federal requirement that would have forced U.S. tobacco companies to put large graphic images on their cigarette packages to show the dangers of smoking, ruling that it violated the First Amendment of the U.S. Constitution that covers free speech.