DENVER — As Democrats from across the country converge here for a convention aimed principally at unifying their party, the most closely watched subplot will be the drama of the Clintons.

Senator Clinton will address the convention in prime time tomorrow and her husband will speak on Wednesday in two of the most anticipated speeches of the four-day gathering. Their goal: to convince Mrs. Clinton’s hold-out supporters and the public at large that their support for Senator Obama’s candidacy is genuine and strong.

With a small but vocal contingent of Clinton backers refusing to support Mr. Obama, concerns persisted yesterday among some Democrats that a plan to place Mrs. Clinton’s name in nomination for a symbolic roll-call vote could expose a lingering rift in the party. Mrs. Clinton appeared to be working behind the scenes to shore up support for Mr. Obama in the hopes of a public display of party unity at the convention.

The chairman of the state delegation, Rep. Charles Rangel of Harlem, told reporters last night that Mrs. Clinton had told him to urge members of the delegation to vote for Mr. Obama — rather than Mrs. Clinton — on the first ballot.

Yet that message had not been spread throughout the delegation as of last night, and at least one New York lawmaker, Rep. Gary Ackerman, said he planned to vote for Mrs. Clinton if her name was placed in nomination. “I’m voting for the favorite daughter,” he said in an interview at a state delegation reception here last night.

As Mrs. Clinton makes a public bid for party unity, the Republicans are moving quickly to sow dissension in the Democratic ranks.

A day after Mr. Obama unveiled Senator Biden of Delaware as his running mate, the McCain campaign launched a television ad titled “Passed Over” that asks why the presumptive Democratic nominee snubbed his chief party rival, Mrs. Clinton, for the vice presidential slot.

“She won millions of votes, but isn’t on his ticket,” a narrator intones. “Why? For speaking the truth on his plans.”

The ad then features images from the Democratic primary, when Mrs. Clinton questioned Mr. Obama about his lack of specificity on policy, his negative attacks on her, and his connection to a convicted Chicago developer, Antoin Rezko.

“The truth hurt, and Obama didn’t like it,” the narrator concludes.

A spokeswoman for Mrs. Clinton, Kathleen Strand, responded yesterday that the former first lady’s support for Mr. Obama is “clear.”

“She has said repeatedly that Barack Obama and she share a commitment to changing the direction of the country, getting us out of Iraq, and expanding access to health care. John McCain doesn’t,” Ms. Strand said. “It’s interesting how those remarks didn’t make it into his ad.”

At the reception last night, several staunch Clinton backers voiced confidence that she and her husband would be able to rally the party behind Mr. Obama.

“The Clintons are great Democrats, and they will do what they have to do to rally the party,” the president of the American Federation of Teachers and a close Clinton ally, Randi Weingarten, said.

At the same time, they conceded that there was little the Clintons could do to sway their most committed supporters who have refused to back Mr. Obama. “There might be a few holdouts,” Rep. Jerrold Nadler said, but he added that he thought the vast majority of Democrats would support the party’s nominee.

Several New York lawmakers who had supported Mrs. Clinton also praised Mr. Obama’s selection of Mr. Biden, saying he had been their second choice, behind Mrs. Clinton, for the no. 2 slot.

In the early hours in Denver, both sides of the debate among Clinton supporters were on display. A handful of her backers were walking down the city’s 16th Street outdoor mall carrying “Hillary” placards and signs urging steadfast support for the former first lady. At the airport yesterday, a young woman wore a button with a more conciliatory message: “Hillary Supports Obama, and So Do I.”

As plans were being finalized for the Clintons’ speeches, Mr. Obama’s chief strategist, David Axelrod, said on ABC’s “This Week” that Mr. Obama and President Clinton had a lengthy conversation by phone on Thursday in what is reported to be their third such discussion since the Illinois senator clinched the party nomination in June.

In addition to her speech tomorrow night, Mrs. Clinton this morning will address a breakfast meeting of the New York delegation. Also expected to be on hand are Senator Schumer, Governor Paterson, and other senior state officials. Mrs. Clinton will also deliver remarks tomorrow to a reception organized by one of her most loyal supporting groups, Emily’s List, and she will attend a health care forum on Wednesday.

Tonight’s convention speeches will feature the would-be first lady, Michelle Obama, along with a tribute to the ailing Senator Kennedy of Massachusetts and remarks by his niece, Caroline Kennedy, daughter of the slain president. There were reports yesterday that Mr. Kennedy, who was diagnosed with a malignant brain tumor earlier this year, would make a surprise appearance at the convention, provided that his doctors give him the go-ahead to fly. He had been expected to address the convention in a videotaped message recorded during the summer.