A slow-moving crisis is approaching for Social Security, threatening to undermine a central pillar in the retirement of tens of millions of Americans.

Next year, for the first time since 1982, the program must start drawing down its assets in order to pay retirees all of the benefits they have been promised, according to the latest government projections.

Unless a political solution is reached, Social Security’s so-called trust funds are expected to be depleted within about 15 years. Then, something that has been unimaginable for decades would be required under current law: Benefit checks for retirees would be cut by about 20 percent across the board.

“Old people not getting the Social Security checks they have been promised? That has been unthinkable in America — and I don’t think it will really happen in the end this time, because it’s just too horrible,” said Alicia Munnell, the director of the Center for Retirement Research at Boston College. “But action has to be taken to prevent it.”