June 4 (Bloomberg)—New York Senator Hillary Clinton is planning to concede the race for the Democratic presidential nomination to Illinois Senator Barack Obama on June 6, a campaign adviser said.

The adviser spoke on the condition of anonymity because Clinton's plans haven't been made public. Her campaign spokesman, Mo Elleithee, would only say that Clinton is "taking the next few days to review her options and talk to party leaders and uncommitted delegates."

Obama, 46, clinched the nomination last night by amassing the required number of delegates to the Democratic national convention in August. Party leaders said they expect Clinton, 60, to voice her support for Obama and help him campaign against presumptive Republican nominee John McCain.

"She's going to endorse him," Clinton adviser James Carville said in an interview. While he said he couldn't confirm Clinton's withdrawal from the race, he said her exit at this point wouldn't be a surprise.

Pressure has been mounting on Clinton, including from some of her supporters, to end her campaign so the Democratic Party can unite in preparation for the general election.

Another string of superdelegates announced their endorsement of Obama today. Among them was New Jersey Senator Frank Lautenberg, who had been one of eight uncommitted Democrats in the Senate. On the House side, Democratic Caucus Chairman Rahm Emanuel of Illinois, former domestic policy adviser to Bill Clinton, also endorsed Obama.

'Full Attention'

Senate Majority Leader Harry Reid, House Speaker Nancy Pelosi, Democratic Governors' Association Chairman Joe Manchin, and Democratic National Committee Chairman Howard Dean pressed the remaining superdelegates to make up their minds.

"Democrats must now turn our full attention to the general election," they said in a joint statement. "We are urging all remaining uncommitted superdelegates to make their decisions known by Friday of this week so that our party can stand united."

Some Clinton supporters expressed frustration that she hasn't bowed out. In an interview with ABC News, Representative Charles Rangel of New York, one of Clinton's staunchest supporters, said it is time for Clinton to publicly state her future plans and allow her supporters to switch their allegiance to Obama.

"Unless she has some good reasons—which I can't think of—I really think we ought to get on with endorsements" of Obama, the New York Democrat said.

In a speech last night after winning the South Dakota primary, and after Obama became the victor in the delegate race, Clinton she wanted to take time to decide what to do.

"This has been a long campaign, and I will be making no decisions tonight," she said in New York.

Many of Clinton's supporters are pushing Obama to pick her as his running mate. She indicated to congressional colleagues yesterday that she is open to the idea.