Senate Democratic leaders on Wednesday proposed a new 5.6% tax on people earning more than $1 million a year to cover the cost of President Barack Obama's $447 billion jobs plan, a move designed to stem Democratic defections from a top White House priority.

The millionaire tax would supplant the tax package Mr. Obama had proposed to offset the jobs bill's cost, which had drawn fire from some fellow Democrats and almost all Republicans. The tax package included limits on upper-income families' deductions...