"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.
It's time to unite the Democratic Party, he said.
"Hillary, of course, will make the decision as to if and when she ends her campaign. But I hope that she reaches that decision soon so that we can concentrate on a unified party capable of winning the White House next November," he said.
