DENVER - Sen. Hillary Clinton takes the stage at the Democratic National Convention Tuesday, a potentially pivotal moment that could help determine whether the party unifies behind Sen. Barack Obama or continues to harbor divisions that might help Republican Sen. John McCain take the White House.

Clinton stressed anew Monday that she supports Obama and wants the 18 million people who voted for her in the primaries to transfer their allegiance as well.

“We are here for one purpose: to give the party momentum going into the general election so that come November, Barack Obama will become the president of the United States,” she told supporters at a Denver hotel Monday.

Yet signs of trouble for Obama persisted heading into her speech. About half of Clinton’s supporters are still not sold on Obama, polls show, with some leaning his way and others saying flat out they’ll vote for McCain.

McCain rushed out a new ad featuring a Clinton supporter saying she’d now vote for the Republican.

“I’m a proud Hillary Clinton Democrat,” said Debra Bartoshevich, a Wisconsin nurse who was removed as a convention delegate after she said she would cast her vote for McCain.

“She had the experience and judgment to be president,” Bartoshevich said in the ad. “Now, in a first for me, I’m supporting a Republican, John McCain. I respect his maverick and independent streak, and now he’s the one with the experience and judgment.”

McCain also dispatched to Denver one of his most prominent women supporters, former Hewlett-Packard chairman and chief executive Carly Fiorina, to drive the point home.

“The Democratic Party is ... still divided,” Fiorina said. “They are not coalesced behind Barack Obama.”

Clinton told delegates Monday that they should ignore McCain’s ad. “John McCain is sending a message,” she said. “I’m here to tell you my name is Hillary Clinton. And I do not support this message.”

A USA Today/Gallup Poll shows that fewer than half of Clinton supporters, 47 percent, are solidly behind Obama. Another 23 percent said they’d support Obama but might change their minds, and 30 percent said they’d vote for McCain.

Obama will need every one of those votes if the November election is as close as the polls now suggest.

“He has to do better,” said Peter Brown, assistant director of the Polling Institute at Quinnipiac University.

“If he wants to win the election, he needs to get the vast majority of Hillary supporters. Her strongest support is where he’s weakest: white, working-class men and women.”

Obama told reporters Monday that he is “absolutely convinced that both Hillary Clinton and Bill Clinton understand the stakes . . . Senator Clinton has pledged wholehearted support to my campaign, as has President Clinton.”

He added that he had considered making Hillary Clinton his running mate before deciding upon Sen. Joseph Biden of Delaware. “I took her very seriously,” Obama said.

To reach them, Obama must use the convention speeches and videos to stress how much he is like those voters - humble beginnings, family life - rather than how much he differs - his mixed cultural background, his living overseas.

“They have to make those voters comfortable with Barack Obama, have to make them think he’s part of the American dream,” Brown said.

And they have to hope that Clinton, and her husband, who speaks on Wednesday, will deliver.

Hillary Clinton, who’d demanded a roll-call vote for the nomination so all her delegates would have a moment in the spotlight, was negotiating a plan that would have her suspend the Wednesday roll call when it got to New York, the state she represents in the Senate, and move to make the nomination unanimous.

Obama’s camp was confident she’d use her Tuesday prime-time speech to deliver an enthusiastic call to support Obama. Ultimately, they said Monday, they think her supporters will come to Obama - though they conceded it might not happen this week.

“There is no stronger surrogate for Sen. Obama among Senator Clinton’s supporters than Senator Clinton,” said senior Obama adviser and family friend Valerie Jarrett.

“So we are confident as we come through this convention that our party will come together ... It might take a little time for some people to come around ... But we can’t let the few voices distort the vast majority of her supporters that I do believe support Senator Obama.”

Added Sen. Dick Durbin, D-Ill.: “Many are coming our way. ... It will take a little longer.”

___

(McClatchy Newspapers intern Jessica Cherry contributed to this report.)