Story highlights At issue is extending an expiring health insurance program for retired miners and their spouses

The House of Representatives sent the bill to the Senate where Democrats are vowing to fight

(CNN) Senate approval of a must-pass short-term government spending bill was threatened late Thursday as West Virginia Sen. Joe Manchin and other Democrats worked to bolster miners' health insurance provisions in it.

They warned they might block the must-pass funding bill if their demands were not met, potentially leading to a government shutdown.

It was not clear if the Democrats could muster the 41 votes they need to block the Republican-authored bill. Adding more uncertainty to the situation, that procedural vote is currently scheduled for Saturday morning, hours after the Friday midnight deadline for when the government would run out of money.

The push is being led by several coal state Democrats up for re-election in two years, and is supported by the Senate Democratic leadership.

At issue is extending an expiring health insurance program for retired miners and their spouses.

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