(Adds comments by Williams, details of election and coming challenges)
DETROIT, June 4 (Reuters) - The United Auto Workers union elected Dennis Williams to a four-year term as its president on Wednesday, putting him in charge of a union facing tricky contract talks with Detroit's "Big Three" automakers next year.
Williams, 61, has said he will serve only a single term as president of the 78-year-old union.
He faces difficult 2015 contract negotiations with the three major U.S. automakers, General Motors Co, Ford Motor Co and Chrysler, a unit of Fiat Chrysler Automobiles , as UAW members will press for the elimination of a two-tiered wage scale.
About a quarter of unionized workers at the U.S. automakers are so-called second-tier employees who make less than veteran workers. The UAW accepted the tiered wage structure in 2007 as the automakers were struggling. Now all three are profitable.
Williams succeeds Bob King, 67, who served a four-year term. Williams will be inaugurated on Thursday at the UAW convention in downtown Detroit.
"The challenges are real, without a doubt," Williams told reporters after his election. He listed the 2016 presidential election, next year's contract negotiations with automakers and the need to close the income gap between Americans.
Williams, an ex-Marine, has been in the UAW since 1977, when he worked at heavy equipment maker J.I. Case factory.
An early supporter of President Barack Obama, Williams said the UAW will work hard to keep the White House in Democratic hands in 2016.
"There's a movement growing in this country and there's a huge dissatisfaction," he told Reuters earlier on Wednesday. "Millionaires and billionaires are growing in their economic statute but the common person is not. We can't continue to allow the middle class to shrink."
Williams also needs to continue the effort led by King to organize foreign-owned automakers with U.S. plants. In February, the UAW lost an election to represent workers at a Volkswagen AG plant in Tennessee.
The UAW has nearly 400,000 members, down from about 655,000 a decade ago and from 1.5 million at its peak in 1979.
Gary Walkowicz, a Ford worker from Dearborn, Michigan, challenged Williams, but received only 49 delegate votes to Williams' tally of 3,215.
Walkowicz said he knew he was not going to win but ran in order to send a message to union leaders that rank-and-file members are tired of giving concessions to financially healthy corporations.
Also elected on Wednesday were Gary Casteel as secretary-treasurer and three vice-presidents: Jimmy Settles, Cindy Estrada and Norwood Jewell.
(Editing by Matthew Lewis)