As President Bush and Congressional Democrats begin negotiations on a package of measures to stimulate the economy, the big fight will be over whether to put extra money in the hands of tens of millions of low-income families who paid little or no income tax last year. Nearly 40 percent of Americans owed no federal income tax last year, though even low-income workers paid taxes for Social Security and Medicare. While Mr. Bush has refused to disclose specifics of his $145 billion plan, administration officials and Republican lawmakers favor a proposal that would offer rebates of up to $800 for individuals and $1,600 for families — but only if they paid that much in taxes last year.