SIOUX FALLS, S.D. (AP) -- Former Sen. George McGovern, an early supporter of Hillary Rodham Clinton, urged her to drop out of the Democratic presidential race and endorsed her rival, Barack Obama.

After watching the returns from the North Carolina and Indiana primaries Tuesday night, McGovern said today it's virtually impossible for Clinton to win the nomination. The 1972 Democratic presidential nominee said he had a call in to former President Clinton to tell him of the decision, adding he remains close friends with the Clintons.

"I will hold them in affection and admiration all of my days," he said of the Clintons.

McGovern's announcement comes a day before Clinton was scheduled to travel to South Dakota to campaign. The state has its primary June 3 with 15 pledged delegates at stake.

McGovern said he had no regrets about endorsing Hillary Clinton months ago, even before the Iowa caucuses.

"She has run a valiant campaign. And she will remain an influential voice in the American future," he said.

McGovern is not a superdelegate.