President Bush has spoken often of adding personal investment accounts to Social Security, but when Democratic challenger John F. Kerry charged this week that the idea's cost would greatly add to the deficit and harm beneficiaries, the Bush campaign quickly cried foul.

"It's a fabrication," campaign policy director Tim Adams said of Kerry's accusations. "They've . . . tortured the numbers and come up with something that's not even real. It's not our proposal."

Technically, Adams is correct that Bush has not offered a specific plan for revamping Social Security. From Social Security to tax reform to abortion, Bush has sounded major policy shifts for a second term, but he has provided no details that could be used by voters to assess the proposals -- or by opponents to attack them.

But that has not stopped Kerry aides from postulating the cost and impact of these proposals, which they have dubbed "phantom proposals."

"Bush is proposing a lot of vague things here," said Brooks Jackson, director of the nonpartisan FactCheck.org.

On Social Security, Kerry aides assume that if Bush is reelected, he would push the second of three policy options detailed by his Social Security Commission in late 2001. That option would divert 2 percent of workers' pay to personal investment accounts, at a cost to the system of about $1.5 trillion over the first 10 years, and a long-term cost to future retirees of as much as 45 percent of their promised benefits, according to the nonpartisan Congressional Budget Office.

On taxes, the Kerry camp has taken Bush's vague pledge to simplify the tax code and decided he wants to introduce a national sales tax, which they say would shift more of the federal tax burden onto the poor and middle class. To fulfill Bush's pledge to respect "the sanctity of life," Kerry insists a Bush second term would see the appointment of socially conservative judges who would eventually overturn the constitutional right to an abortion.

"Their goal is to get a mandate without explaining what it is they're trying to get a mandate for," said Jason Furman, the Kerry campaign's economic policy director, "and our goal is to pin them down on what they want a mandate for."

That strategy has come at a cost, however. The president's campaign has repeatedly challenged the Kerry team's assumptions, and some media outlets and independent analysts have castigated Kerry for what they see as the Democrat's distortions. Joe Lockhart, a top Kerry media strategist, said, "If they're not going to put numbers on the programs, then we'll do it for them."

For his part, Kerry has failed to detail how he would address the budget deficit or tackle looming crises in Social Security and Medicare. But he has laid out in significant detail his signature health insurance proposal, his plans to raise some taxes and change the way corporations are taxed abroad, his education proposals, and other domestic policy ideas.

Adams noted that Kerry provided most of his details during the primary season, when core Democratic voters were demanding to know more about where the candidates stood and the candidates had the luxury of offering policy details on a relatively small political stage.

"We have to be more generalized [in describing proposals] based on the nature of where we are in the campaign," Adams said, adding that, "it's tough to get into the specifics and details of policy, and get a stadium of 30,000 on their feet."

But there are other advantages to avoiding the nitty-gritty. Without specifics, Bush has been able to sound popular themes -- such as allowing younger workers to invest their tax dollars in stocks and bonds, respecting the "sanctity of life," or making the tax code simpler and fairer -- without divulging any of the costs, critics charge.

"It's a shrewd campaign ploy," said Robert Dallek, a Boston University presidential scholar and biographer.

Candidates running on similarly dramatic themes have taken divergent tacks, Dallek said. Bush's strategy is reminiscent of Franklin Delano Roosevelt's 1932 campaign, when he pledged a "New Deal" for Depression-era America, while providing scant detail and promising to balance the budget. His opponent, President Herbert Hoover, called him "a chameleon on plaid."

But Bush has left voters to guess whether he intends to "transform" what he called "many of our most fundamental systems," or whether "this is little more than a political ploy," Dallek said.

Social Security may be the clearest example. For more than four years, Bush has advocated the diversion of some Social Security taxes into personal investment accounts. But the details have been elusive. Would the president divert 2 percent of a worker's pay, as more traditional privatization models suggest? Or would he embrace new, more sweeping -- and possibly disruptive -- models, that would divert 6 percent, nearly half of all Social Security taxes?

A 2 percent diversion would take about $1.5 trillion out of the Social Security system, according to the CBO, and that money would have to be found elsewhere if current Social Security recipients are to receive their full benefits, as Bush has promised. If he opted for a 6 percent diversion, those 10-year costs would exceed $4 trillion. Would the president borrow that money, potentially doubling the $7.4 trillion federal debt, or would he cut spending and raise taxes to finance the plan?

Answering such questions would only work against the administration as it seeks compromises with Congress, said Adams, who preferred a "conceptual approach."

"Social Security still remains a politically charged issue, and by delving into too much detail now, you undermine your ability to achieve a legislative accomplishment later," he said.