DENVER — For a judge who would go on to make same-sex marriage legal in Utah, a deep-red state where streets in the capital are numbered by their distance from the Mormon temple, Robert J. Shelby arrived on the bench with enthusiastic praise from Republican leaders.

He had been a combat engineer in the Persian Gulf conflict and was, according to state voter records, a registered Republican. Senator Orrin G. Hatch, a seven-term Utah Republican, recommended him for a federal judgeship, calling him an experienced lawyer "with an unwavering commitment to the law." Senator Mike Lee, a Tea Party Republican, said that Mr. Shelby was "pre-eminently qualified" and predicted he would be an outstanding judge.

Now, less than two years since he joined the bench, the same-sex marriage case has transformed Judge Shelby into a hero for hundreds of newlywed gay couples and an object of derision for many social conservatives who supported Utah's 2004 ban on such unions.

Gov. Gary R. Herbert, a Republican, called him an "activist federal judge," and state lawyers are already trying to have higher courts roll back his decision, potentially undoing as many as 900 new same-sex marriages.

State officials are expected to ask the United States Supreme Court as early as Monday to halt the waves of same-sex unions that began minutes after Judge Shelby handed down his decision on Dec. 20 that Utah's measure barring same-sex marriage violated the United States Constitution. Utah leaders say the rush of marriages has sown "chaos," and say they should be halted until the legal case is resolved. Judge Shelby has already refused to stay his own decision, and the United States Court of Appeals for the 10th Circuit also denied Utah's request to stop the marriages.

Utah's request will initially land with Justice Sonia Sotomayor, who oversees the 10th Circuit. She is expected to refer it to the entire court, and the justices could rule within days.

It is an unlikely cacophony created by a man who quietly ascended through Utah's legal ranks, mostly avoiding media attention, law review articles and other totems of success in the legal profession.

Born in Fort Atkinson, Wis., Judge Shelby, 43, moved to Utah for college and now lives in Salt Lake City with his wife and their two children. As a young man, he worked for Snappy Car Rental and was a night manager at a Maceys grocery store in Logan, Utah.

He spent his legal career in private practice, working on white-collar criminal defense, commercial law and serious personal injury lawsuits. He was a member of the defense team representing Olympic officials in Salt Lake City accused of bribery in the city's efforts to win the 2002 Winter Games. In his Senate confirmation questions, he said he did free legal work for poorer criminal defendants — usually about two cases a year.

"So many people thought he was so good, just brilliant," said Andrew Morse, president of Snow, Christensen and Martineau, the firm where Judge Shelby had been a shareholder when he was nominated to the bench. "He's just easy to get along with, always says the right thing."

He was active in the state and local bar associations, and was a leader in a group that mentors young trial lawyers. But he largely stayed out of the spotlight and away from politics. In 2010, he and his wife gave a $50 contribution to a Republican candidate for the State Legislature — the only political donation found in state and national campaign-finance databases.

Former colleagues said they did not know his religious background. The judge did not respond to emails and a phone call requesting an interview.

His answers to questions about empathy in judges, and whether judges should protect the "little guy," are as mild as a glass of milk. "I believe it is the judge's responsibility to reach decisions based exclusively on the application of established precedent to the specific facts presented," he wrote.

But this month, when it was time to decide a case brought by three gay couples challenging Utah's ban on same-sex marriages, Judge Shelby appeared to be moved by their stories. In his 50-page ruling, he repeated the stories of how each couple met and fell in love, referring to them not only as plaintiffs or by last name, but as Derek and Moudi, Karen and Kate, Laurie and Kody.

He wrote that Utah's voter-approved ban on same-sex marriage violated the Constitution's guarantees of equal protection under the law, and said that the ban denied gay and lesbian couples "their fundamental right to marry."