Independent Senator Bernie Sanders is proposing to expand the estate tax on wealthy Americans, including a rate of up to 77 percent on the value of estates above $1 billion.

Sanders of Vermont, who is considering a second run for president, said in a statement his plan would apply to the wealthiest 0.2 percent of Americans. It would set a 45 percent tax on the value of estates between $3.5 million and $10 million, increasing gradually to 77 percent for...