BOSTON – Democrats itching to retake the White House open their convention here tonight with an attempt to graft a little Clinton charisma onto the party’s expected nominee for president, Senator Kerry of Massachusetts.

President Clinton and Senator Clinton are to address the delegates in prime-time speeches that all the major television networks plan to carry live.

The high-profile showcase for the two Democratic heavyweights is fraught with promise and peril for the party and for Mr. Kerry. Party leaders say the Clintons must make a compelling case for Mr. Kerry while not evoking such a thunderous response that Mr. Kerry suffers by comparison. Mr. Kerry’s reputation for being aloof and less than empathetic in his public persona makes the Clintons’ task even more difficult.

Yesterday, at a reception sponsored by Jewish groups, Mrs. Clinton indicated she understood that many people see Mr. Kerry as less affable than President Bush.

“Sometimes in polls people are asked, ‘When it comes to choosing a president, well, who would you like to have a beer with?'” Mrs. Clinton said. “You know, if that was ever the right question, it no longer is.”

Mrs. Clinton said that with the nation entangled in Iraq and facing a host of serious national security threats, voters should set aside any concerns about personality. She described Mr. Kerry as “a serious man for a serious job.”

During convention planning, the Kerry campaign snubbed Mrs. Clinton. It initially planned to have her stand silently on the podium tonight, as Senator Mikulski of Maryland paid tribute to women in the Senate.

After a flurry of embarrassing press accounts and complaints from some prominent women in the party, Mr. Kerry invited Mrs. Clinton to introduce her husband.

Mr. Kerry’s nomination on Thursday night is a foregone conclusion. The more than 4,000 delegates are not expected to debate any substantive issues over the next four days, but Democratic leaders are convinced the convention will have a critical impact on undecided voters who are unhappy with Mr. Bush but not quite comfortable with Mr. Kerry.

“This is when America starts really paying attention,” Mrs. Clinton said. “The convention and the debates will be probably more important in deciding the outcome this time than any time in recent memory.”

Senator Schumer declared yesterday that Mr. Kerry’s speech Thursday would be the most important of his career.

“This is the first time the average American will see him,” said Mr. Schumer. “Thursday night is make or break.”

Mere months after a former Vermont governor, Howard Dean, electrified the Democratic presidential race with a message of anger at Mr. Bush and the Iraq war, party leaders called on Mr. Kerry to eschew attacks against the president and to exude a sunny optimism.

The attorney general of New York, Eliot Spitzer, called on Mr. Kerry to “craft a positive message.”

“Anger is not a useful emotion,” he said. “We will win based on the affirmative argument that John Kerry makes on Thursday night.”

Noting that this is the first political convention since the September 11, 2001, attacks, Senator Lieberman of Connecticut predicted that “allies and enemies around the world” would be watching the addresses by Mr. Kerry and his running mate, Senator Edwards of North Carolina. Mr. Lieberman urged the men to “speak and act in a way that gives the American people confidence.”

“In the end, you don’t win by being against something. The American people are tired of back-biting and negativism,” Mr. Lieberman said.

“He has to start persuading people why he would do a better job than Bush in protecting people,” said Rep. Jerrold Nadler, a Democrat of New York. “If he does that, this will be a runaway victory,” he said.

An early supporter of Mr. Kerry, Rep. Gregory Meeks, a Democrat of New York, will be opening the convention. He said Mr. Kerry must communicate that he can “bring this country together and to lead. He’s got to demonstrate what he did in Vietnam.”

The mayor of Boston, Thomas Menino, welcomed the New York delegates at a beachside party in South Boston, instructing the politicians and activists to “enjoy yourselves.”

Asked to offer advice to New Yorkers as they prepare to host the Republican National Convention in late August, Mr. Menino said authorities should spell out to the public all the inconveniences and disruptions they will experience.

“Just tell the public what is really going to happen,” he said, citing traffic and security preparations.

“Get it out there. No surprises. It hurts a little bit at first, but it helps in the long run,” he said, and later posed for pictures next to a sand replica of the New York skyline.

While the Republican convention in New York is expected to draw tens of thousands of protesters, the Democratic gathering has been less of a magnet for demonstrators. Yesterday afternoon more than a thousand people marched through downtown Boston past the convention site. The motley crowd represented various groups and causes, though a unifying theme appeared to be staunch opposition to the war in Iraq.

“I’m against the war. I think it’s a racist war and I’m against racism,” said a 55-year-old social worker from Brooklyn, Anne Pruden. “The Democrats are selling us out.”

Asked about Mr. Kerry, Ms. Pruden said, “He doesn’t represent my interests. He’s a rich dude.” She said she would vote for “an independent candidate.”

One protester said he had more than a passing acquaintance with Mr. Kerry. A construction worker from Levittown, Pa., William Perry, said he worked with Mr. Kerry on several occasions in the early 1970s while both men were active in Vietnam Veterans Against the War.

Mr. Perry carried banner that read, “How do you ask a soldier to be the last person to die for a lie?” The sign loosely paraphrased Mr. Kerry’s famous testimony to the Senate Foreign Relations Committee in 1971.

Mr. Perry called on Mr. Kerry “to put forward an immediate withdrawal plan for Iraq within 10 days of his inauguration.”

Mr. Kerry has said he plans to keep troops in Iraq indefinitely, but Mr. Perry said he thought the senator might be hiding his true intentions.

“He’s not much different from Bush presently, but that is for mass consumption,” Mr. Perry said. “I’m hoping he can be moved.”

Mr. Perry looked exasperated when asked if he would vote for Mr. Kerry in November. “Don’t make me say it. It’s not fair. What choice do I have? I got to vote for the guy,” Mr. Perry said.

Here as in New York, there has been litigation over the rights of protesters to gather in close range to the delegates and the convention venue. Security officials here set up a demonstration zone near a parking lot where delegates are expected to arrive to enter the convention hall. Lawyers for protesters challenged the plan to confine large demonstrations to the penned-in area, which sits under an overhead subway line that is no longer in use.

In an unusual opinion issued on Friday, a federal judge, Douglas Woodlock, harshly decried the conditions in the protest zone but effectively upheld the city’s right to force demonstrators to use it.

The judge called the demonstration zone “an offense to the spirit of the First Amendment.”

“It is a brutish and potentially unsafe place for citizens who wish to exercise their First Amendment rights,” Judge Woodlock wrote. He noted that the protest area contains various hazards and obstructions.

The judge also said the facility, which is covered with mesh and nearly surrounded by opaque 8-foot-high fences, appeared to violate building codes.

“One cannot conceive of what other design elements could be put into a space to create more of a symbolic affront to the role of free expression,” Judge Woodlock said. He said that the precautions appeared to be lawful because they were required to protect delegates from being pelted with rocks or sprayed with bleach or urine.

The judge did order the city to allow yesterday’s protest march.