Rep. James L. Oberstar (D-Minn.), right, talks with Democratic presidential candidate Walter Mondale in Gilbert, Minn., in 1983. (Jim Mone/AP)

James L. Oberstar, the son of a miner who became a power in Washington during his 36 years in the House, where he was chairman of the Transportation Committee, died May 3 at his home in Potomac, Md. He was 79.

The family issued a statement saying the cause of death was not known.

Mr. Oberstar, who was the ­longest-serving congressman in Minnesota history, was first elected in 1974. His legacy is visible throughout his home state, where his Washington influence secured funding for public works projects including the United States Hockey Hall of Fame in Eveleth, a commuter rail system in the Twin Cities and a state-of-the-art water treatment plant in Ely.

Although he was a prolific user of “earmarks,” or federal spending tailored to particular projects, Mr. Oberstar had a reputation as a serious and hard-nosed expert in public works and transportation issues. He is remembered in polarized Washington as a lawmaker deeply interested in policy who maintained warm, even close relationships with his ideological opponents.

“He was the leading infrastructure expert of our time,” said Vin Weber, a former Republican congressman from Minnesota who served with Mr. Oberstar and developed a friendship despite partisan differences.

Weber, like other friends of Mr. Oberstar, was stunned at the congressman’s death because he had been in good health and was an avid bicyclist. Mr. Oberstar apparently died in his sleep with no warning, friends said.

Weber recalled that Mr. Oberstar was held in high regard by Republicans because he sought to keep issues before the Transportation Committee free of partisan rancor. Weber, a leading conservative, bought a house in Mr. Oberstar’s district and confessed that the pro-labor Democrat won his support.

“My wife and I usually voted for him,” Weber said. Mr. Oberstar was elected to 18 successive terms.

While in Congress, Mr. Oberstar was a generally reliable left-leaning vote. He backed the stimulus program in 2009 that brought increased demand for steel from the Mesabi Iron Range region where he grew up. He was, however, a centrist in the Minnesota Democratic-Farmer-Labor Party. For much of his career, he had support from antiabortion activists and the National Rifle Association.

In the world of transportation, Mr. Oberstar had an international reputation as an expert and as an advocate of public investments to spur private growth. He became known, among other things, as a champion of “intermodality,” the idea of linking highway, air and rail systems with urban buses, subways and bike paths, for which he was an ardent champion.

The political strength Mr. Oberstar gained became his weakness in 2010 when he lost narrowly to GOP challenger Chip Cravaack, who campaigned against business as usual in Washington, including the use of earmarks to deliver pork-barrel projects back home.

The loss was stunning for Mr. Oberstar, who had won reelection in 2008 with 68 percent of the vote. He had amassed a large campaign war chest but had given much of it away because he thought his seat was safe.

He had been unprepared for two developments that swept Republicans to power in the House in 2010. First, the U.S. Supreme Court’s decision on Citizens United v. Federal Election Commission that year permitted almost unlimited election-related spending by labor unions, individuals and corporations in remote districts like his.

Second, Mr. Oberstar was surprised by the populist anger directed at establishment politicians and practices, such as earmarking legislation, that had been used for years to stimulate the economy in his mostly rural district.

When asked about his upset defeat in 2010, Mr. Oberstar told a local reporter: “I go in peace of mind and heart, but with sadness.”

James Louis Oberstar was born Sept. 10, 1934, in Chisholm, a small city on the Iron Range of northeastern Minnesota. He was the son of an Iron Range miner.

Mr. Oberstar excelled at school and graduated in 1956 from the College of St. Thomas in St. Paul, Minn. A year later, he received a master’s degree in European studies from the College of Europe in Bruges, Belgium. He spoke a half-dozen languages, including Creole, which he learned with such fluency that he was briefly hired to teach the language to U.S. Marine and Navy officers assigned to Haiti.

Mr. Oberstar came to Washington in 1963 to work for his hometown congressman, John Blatnik (D), who was then a powerful leader of the House Public Works Committee. While working for Blatnik, Mr. Oberstar learned the politics of earmarking and the subtleties of transportation and infrastructure spending.

In 1963, he married Jo Garlick. After her death in 1991, he married Jean Kurth in 1993. Survivors include four children from his first marriage; two stepchildren; and eight grandchildren.

Sen. Al Franken (D-Minn.) recalled seeing Mr. Oberstar chair a hearing on high-speed rail with witnesses from France, Japan and other countries with advanced rail systems.

“When it came time for the chairman to ask his questions, I learned that Jim had piloted every one of these high-speed trains,” Franken recalled. “Jim questioned the French witness in startlingly fluent French. It was a tour de force. I think that’s French.”

Mr. Oberstar broke in to French or Creole with the slightest provocation. But he was not pompous or humorless. When he was elected to Congress in 1974 and joined the Public Works Committee as a junior member, he was invited to give an opening statement.

He acknowledged to his fellow committee members that his new elected status came with something of a diminution of the power he had known when he served as a staffer to Blatnik, the previous committee chairman.

“Well,” Mr. Oberstar began, according to an article about his freshman remarks in Congressional Quarterly Magazine, “this is a very different position from when I had real power on this committee.”

Colby Itkowitz and Alice Crites contributed to this report.