Former president Bill Clinton, in phone calls with John F. Kerry over the weekend, told the Democratic presidential nominee that he must sharpen his criticism of President Bush's record and offer voters a more compelling case in his own behalf if he hopes to win the election in November.

The conversations came as Kerry approved a series of personnel moves that will elevate two of his longtime Boston political allies to senior positions to go along with recently enhanced roles for several former Clinton advisers.

Kerry spoke with Clinton, who is at a New York hospital awaiting heart surgery, on Friday and Saturday. Clinton added his voice to that of other Democrats who were urging Kerry to change his message and strategy in response to gains Bush made during August and at his party's New York convention last week.

Clinton, according to those familiar with the conversation, urged Kerry to draw a sharper contrast with Bush and to explain to voters the effect of going to war in Iraq on domestic policies.

"He always felt that you've got to give people a reason to vote for you and give people a choice," said one Democratic strategist. "He believes that at the end of the day that if you do make it an effective choice for the voters, they'll figure it out. But the burden's on the candidate to make the case."

The Saturday call was first reported by the New York Times.

On Sunday, campaign officials said John Sasso, who has been running the general election operations at the Democratic National Committee, will shift roles to become Kerry's senior adviser aboard the candidate's campaign plane. Michael Whouley, who helped rescue Kerry's campaign in Iowa during the nomination battle, will take over Sasso's responsibilities at the DNC, reprising the role he played for Al Gore four years ago.

In addition, Kerry's campaign has added several former top Clinton advisers in recent days. Former Clinton press secretary Joe Lockhart and presidential assistant Joel Johnson formally joined the campaign last week to strengthen communications and rapid response operations. Doug Sosnik, former White House political director was brought in as a senior strategist.

Democratic sources said that former Clinton adviser Paul Begala, who has been offering regular advice, may play a stepped-up role, but a Kerry campaign official said that Begala has no formal role.

James Carville, who was Clinton's top strategist in 1992, has been urging the campaign to give Begala a more significant role, and a campaign official said Carville will continue as an outside adviser.

Kerry advisers described the moves as long planned and part of an overall effort to put the strongest possible team together for the final 60 days of the campaign. But the decisions on Sasso and Whouley caught some staff members at the campaign and DNC by surprise and, along with the recruitment of the Clinton advisers, were seen by other Democrats as an acknowledgment by Kerry that his campaign needed help after the most difficult month he has endured since winning the nomination in the spring.

Those Democrats said Kerry had been slow to respond to criticisms and that the campaign appeared to be sluggish in some of its decision-making. They predicted that the changes would bring greater focus to the Democratic campaign.

Kerry advisers said they were planning a much more aggressive campaign in the final two months, with Kerry and vice presidential nominee John Edwards leading the attack, and that restructuring at the campaign and the DNC will help in drawing a sharper contrast with Bush and Vice President Cheney.

Since his party's convention in Boston in late July, Kerry has been on a downward slide, attacked by a group of Swift boat veterans over his service in Vietnam and his antiwar protests when he returned to the United States. He was also hurt by several statements that sowed confusion about where he stands on the war in Iraq.

Bush took advantage of both problems and then used the Republican convention in New York last week to hammer Kerry as unreliable on national defense and to present himself as the candidate best able to keep the country safe against terrorism.

The first two polls taken after Bush's convention, by Time and Newsweek, showed an immediate boost, with Kerry trailing by 11 percentage points in both -- the largest deficit either candidate has suffered in the campaign. Kerry campaign officials said their internal polls also put Bush in the lead, but by a narrower margin.

Some of Bush's gains likely will recede quickly, given the normal rhythms of presidential campaigns, but Kerry's performance in August unnerved many Democrats outside the campaign, who groused privately and sometimes publicly that the candidate needed to make a significant mid-course correction to counter Bush's gains.

"This is a critical decision because John Sasso has enormous credibility, and all of a sudden the center of power moves from the ground to the plane, where decisions can be made quickly," said Tony Coelho, who managed Gore's 2000 campaign for a time. "It tells me we have an independent thinker with Kerry who can get things done."

"We need someone on the plane who has a relationship with him and is an adult," said a senior campaign official, who asked for anonymity to talk more freely about strategy. "You need heavier fire power here. There is no room for error in days to come, and this takes the pressure off of headquarters."

Communications director Stephanie Cutter said Mary Beth Cahill, who was brought in last fall at a time when Kerry's candidacy was floundering, would remain in overall control of the campaign, with Sasso adding a heavyweight voice from the campaign trail. "This is the home stretch, and John Kerry wants more of the best and brightest," Cutter said.

Whouley, who like Sasso has a long relationship with Kerry, has broad experience in state-by-state targeting, Electoral College strategy and voter mobilization. Sasso was a top adviser to then-Massachusetts Gov. Michael Dukakis, who ran for president in 1988.

In a previously reported move, Howard Wolfson, a former top aide to Hillary Rodham Clinton and former executive director of the Democratic Congressional Campaign Committee, will join the DNC as senior adviser for communications.

On the Sunday talk shows, Kerry aides and other Democrats sought to play down Bush's post-convention bounce. "We always knew that August was going to be the toughest month for us," Cahill said on CBS's "Face the Nation." "But from here on in, we're in the general election."

Balz reported from Washington.