UPDATE 1-Delaware senator next to face challenge from progressive rival

(Adds academic comment, Biden endorsement, background)

Sept 6 (Reuters) - The potential for another takedown of a longtime Democratic incumbent by a progressive challenger looms in Delaware, where voters on Thursday were choosing between a three-term U.S. Senator and a community activist who has never held office.

Senator Tom Carper, a 71-year-old white man, is being challenged by Kerri Evelyn Harris, 38, who would be the first black woman and openly gay person elected by the state's voters if she were to win in the Nov. 6 general election.

Carper has won a dozen statewide elections over his 40-year career and has faced only token opposition in Senate nominating contests in the past.

The race is the latest test of whether voters dissatisfied with Democratic leadership will push out an incumbent in favor of a younger, more diverse candidate they see as a potentially more robust opposition to Republican President Donald Trump.

"She has run a strong campaign. I do think we're going to see a closer race than I could have imagined when this all started," said David Redlawsk, chair of the political science department at the University of Delaware.

The contest follows June's shock win by Alexandria Ocasio-Cortez over high-ranking Representative Joe Crowley in a New York City congressional district and Tuesday's upset by Boston city councilwoman Ayanna Pressley over longtime incumbent U.S. Representative Michael Capuano.

The Delaware vote differs from those primary elections, however, in that it is a statewide race in a place that tends to elect moderates to federal office. The congressional districts expected to soon be represented by Ocasio-Cortez and by Pressley are more liberal than the country as a whole. Pressley is running unopposed.

Political analysts see Carper, a moderate, as a safe win for Democrats in November. A primary victory by Harris on Thursday would likely make the general election race more competitive.

Ocasio-Cortez has held rallies for Harris, an Air Force veteran who has been far outspent. Her campaign's expenditures total a mere $69,000, while Carper has spent $3.35 million, according to the U.S. Federal Elections Commission database.

Carper was endorsed by former Vice President Joseph Biden, who served with Carper in the Senate representing Delaware.

Polling in the Senate race has been limited, with one poll from July showing Carper ahead by more than 30 points.

"It won't be very many voters, and this is a primary, so the unthinkable could certainly happen," Redlawsk said.

Three Republicans are running in Thursday's primary in a state whose last Republican senator, William Roth, lost a 2000 reelection bid to Carper.

An incumbent senator has not lost a primary since 2012, when Indiana Republican Richard Lugar lost to Richard Mourdock, who went on to lose to Democrat Joe Donnelly.

Most of the nation's nominating contests for this election cycle have been completed. Next week features elections where incumbent Democratic governors also face insurgents, in Rhode Island, and most notably New York, where actress Cynthia Nixon is challenging two-term incumbent Andrew Cuomo. (Reporting By David Gaffen in New York Editing by Colleen Jenkins and Bill Berkrot)