By Tim Ghianni

NASHVILLE, Tenn. (Reuters) — Fred Thompson, a former Republican U.S. senator from Tennessee who briefly ran for president and straddled the world of politics and entertainment with a prolific television and film acting career, died of cancer on Sunday at age 73.

Thompson, a onetime real-life federal prosecutor best known to prime-time TV audiences for his role as a district attorney on NBC’s hit show Law & Order, died from a recurrence of lymphoma, said Brent Leatherwood, executive director of the Tennessee Republican Party.

The actor-politician, who first made a name for himself in Washington as a Watergate investigator, was in hospice care at the time of his death, Leatherwood said.

“It is with a heavy heart and a deep sense of grief that we share the passing of our brother, husband, father, and grandfather, who died peacefully in Nashville surrounded by his family,” relatives said in a statement published by the Nashville Tennessean newspaper.

The tall, imposing Thompson was elected to the Senate in 1994 and served for two terms before retiring in 2003.

He announced his bid for the Republican presidential nomination on NBC’s The Tonight Show with Jay Leno in September 2007 but dropped out of the race in January after garnering little support.

Born in Sheffield, Alabama, Thompson earned a law degree from Vanderbilt University, became a federal prosecutor and went to work for longtime Tennessee Republican Senator Howard Baker.

It was Baker who secured him a job as minority legal counsel for the special Senate Watergate Committee investigating the 1972 burglary at the Democratic Party headquarters in Washington.

Thompson has been credited with helping Baker craft the famous question that framed the panel’s inquiry — “What did the president know, and when did he know it?”

During a nationally televised hearing in July 1973, Thompson asked the question that led to the public disclosure by White House aide Alexander Butterfield of a secret Oval Office tape-recording system.

The existence of the tapes played a pivotal role in the investigation of the Watergate cover-up and the 1974 resignation of President Richard Nixon.

Moving between politics and entertainment, Thompson played supporting roles in numerous films, including No Way Out, The Hunt for Red October, Days of Thunder, Die Hard 2 and In the Line of Fire.

His most notable TV role was as the no-nonsense New York D.A. Arthur Branch in the long-running Law & Order series, in which he appeared from 2002 to 2007.

(Additional reporting by Peter Cooney in Washington; Writing by Steve Gorman; Editing by Andrew Hay and Eric Walsh)

Photo: Former U.S. Republican Party Presidential candidate, former Senator Fred Thompson (R-TN), waves as he leaves the Republican party debate in Johnston, Iowa, in this file photo taken on December 12, 2007. REUTERS/Keith Bedford