Coal companies had been required to pay a $1.10 per ton excise tax on underground coal production to finance the federal Black Lung Disability Trust Fund, but the amount reverted to the 1977 level of 50 cents this year after Congress declined to take action to maintain the rate.

The coal industry had advocated for allowing the rate to drop as scheduled, arguing the industry was already facing economic pain and that maintaining the rate was not required to cover the cost of support for afflicted miners.

But the Government Accountability Office has said the fund is now at risk of insolvency, due to soaring debt, a wave of coal company bankruptcies, and a resurgence of the disease that had been nearly wiped out two decades ago.

The fund is intended for disabled miners whose employers go bankrupt and can no longer pay out medical benefits.

The miners and their families traveled by bus from southwestern Virginia, West Virginia and eastern Kentucky to call on lawmakers to restore and extend by 10 years the higher coal excise rate. They were due to meet with Democratic Pennsylvania Senator Bob Casey and Republican Senate Leader Mitch McConnell.

"All these coal companies are filing for bankruptcy and are walking away free - they don't pay their liabilities," said Patty Amburgey, the widow of a black lung victim from Letcher County, Kentucky. "It's time they hear coal miners' voices to hear the pain they are in."

McConnell spokeswoman Stephanie Penn said to Reuters in a statement that benefits are still paying out despite the lower tax rate.

Cindy Brown Barnes, the GAO's director of workforce, education and income security, told Reuters recent bankruptcies "increase the fiscal exposure of this fund." This year alone, five coal companies have gone bankrupt, putting hundreds of miners out of work and raising questions about what liabilities they will be on the hook to cover.

The fund has already been forced to borrow more than $6 billion from the US Treasury to finance benefits during the life of the program, according to the Treasury Department. More than half of the fund's revenue now goes to servicing that debt.