WASHINGTON — An election-year bill to fulfill a 70-year-old government promise and protect health-care and pension benefits for retired miners is dividing coal-state Republicans, pitting endangered incumbents against GOP leaders wary of bailing out union workers.

Retirement and health-care funds currently support about 120,000 former miners and their families nationwide. But account balances have dwindled amid continued layoffs and bankruptcy filings as the coal industry struggles against competition from cheaper natural gas and tightening environmental regulations.

The bill would ensure that retired miners receive hundreds of millions of dollars in benefits now at risk amid the industry’s steep decline. Without congressional intervention, some of the funds could run out of cash by next year, according to the United Mine Workers of America.

“These folks spent their whole lives going underground and mining the coal that made this country what it is today. They energized this nation for decades and through today,” said Phil Smith, a spokesman for the UMW, which has pushed for the bill.

In 1946, President Harry S. Truman brokered an agreement to guarantee miners’ lifetime health and retirement benefits, a move that averted a lengthy strike.

Thousands of retired miners and their supporters are expected to gather at the Capitol Thursday to push for the bill, which they describe as a life-saving measure that honors the pact made by the federal government. The bill would affect more than 30,000 retired miners in West Virginia, and tens of thousands more in Pennsylvania, Ohio and other states.

Republican Sens. Rob Portman of Ohio and Mark Kirk of Illinois — both in tough re-elections in two months — back the legislation, but face resistance from Senate Majority Leader Mitch McConnell of Kentucky. McConnell, a staunch defender of his home state’s coal industry, blocked the pension measure last year and says he’s not going to fast-track a plan that some Republicans warn amounts to a bailout.

“There are hundreds of private-sector pension plans in critical, endangered or declining status throughout America today,” Sen. Mike Enzi, R-Wyo., chairman of the Senate Budget Committee, said earlier this year. “So I’m not sure how Congress would help the United Mine Workers and not the others. Where do we draw the line?”

Wyoming is the nation’s top coal producer — mainly from non-union plants — and Enzi said the bill would “do absolutely nothing for miners who are not members” of the UMW. Nearly 11,000 coal workers have lost jobs in the last year, according to the Bureau of Labor Statistics, and the pension bill “wouldn’t help put those folks back to work,” Enzi said.

In Pennsylvania, Sen. Pat Toomey, another endangered Republican, declined to comment, although his Democratic challenger, Katie McGinty, supports the bill.

“Our nation owes every single coal miner a debt of gratitude for staring down risks and helping meet our energy needs,” McGinty said. She called on Congress to ensure “America’s coal miners get the dignified, secure retirement they deserve.”

A spokesman for McConnell said the senator “has been and remains committed to helping ensure the retirement security of our nation’s retirees, including coal miners.”