Clinton-backer George McGovern tells his candidate it's time to call it quits:

After watching the returns from the North Carolina and Indiana primaries Tuesday night, McGovern said Wednesday 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 that 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 holds 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.

But Obama has won the nomination "by any practical test" and is very close to a majority of the pledged delegates, said McGovern, who is 85. Obama moved within 200 delegates of clinching the nomination with his split decision on Tuesday of a win in North Carolina and a narrow loss in Indiana.