MUSKEGON, Mich. -- President Bush chided Senator John F. Kerry and fellow Democrats yesterday for asserting that Republicans will undermine Social Security, calling the strategy ''the most tired, pathetic way to campaign for the presidency."

Traveling by bus through the southwest corner of this battleground state, Bush tried to improve voters' perceptions of his domestic policies by condemning Democrats for going negative -- even as he held Kerry's plans up to the harshest possible light.

''I'm running against a fellow who has got a massive, complicated blueprint to have our government take over the decision-making in health care," the president said. ''Not only is his plan going to increase the power of bureaucrats in your life, but he can't pay for it unless he raises your taxes.

''What would you expect from a senator from Massachusetts?" Bush said, as a partisan crowd cheered the reference to Kerry's home state and its liberal leanings.

Kerry's campaign said Bush mischaracterized its health care policy, and the president's plans would indeed imperil Social Security.

Bush made three stops during a six-hour bus tour in Michigan, a state he narrowly lost in 2000, before heading to Colorado last night. Polls suggest Kerry is clinging to a slight lead in Michigan, with job losses and health care costs worrying voters.

''Of the 18 percent of voters who are not 100 percent committed to either candidate, their number one issue is health care," said Michigan pollster Ed Sarpolus. The bulk of those wavering voters are elderly women, Sarpolus said, which might explain Bush's posture on Social Security.

Bush said: ''You'll hear the same rhetoric you hear every campaign, believe me -- 'They're going to take away Social Security checks.' It's the most tired, pathetic way to campaign for the presidency."

Kerry spokesman Phil Singer responded: ''The only thing that's tired and pathetic is George Bush's warmed-over Social Security-privatization plan from 2000 that jeopardizes the program, cuts benefits, and results in a $2 trillion deficit."

The senator's backers made their presence known along Bush's bus route. ''Show us jobs," read a sign at the first intersection on the way out of Muskegon.

The White House countered with some theatrics: The president's bus drove into left field at a baseball stadium in Battle Creek, Mich., stopping at the edge of the crowd so Bush could jump out and shake hands along a raised platform.

Kerry plans to expand the health insurance system for federal employees to private citizens through tax credits and subsidies. The government would help companies and insurers pay an employee's catastrophic medical costs if the firms agreed to hold down premiums.

Outside analysts peg the cost at $895 billion over 10 years to cover 27 million more people. Kerry said he would pay for it by rolling back Bush's tax cuts for people earning more than $200,000 a year.

The number of Americans without health insurance has risen during Bush's presidency, reaching nearly 45 million in 2003. Medicare costs are rising rapidly.

On Social Security, the president wants to give younger workers the option of putting part of their payroll tax into personal retirement accounts, giving them a chance to make a higher return.

Bush is scheduled to address members of the National Guard in Las Vegas tomorrow. The president plans to say he's proud of his own service in the Texas Air National Guard, but he won't mention the controversy regarding his Vietnam-era tour, aides said.