Congress has passed on its last chance to help blue-collar manufacturers of geothermal pumps and fuel cells by making a correction — promised for a year — to a spending bill screwup.

The trouble started in December 2015, when Congress was trying to pass an end-of-year tax and spending bill to run the government for the following year.

Republicans and Democrats struck a deal: In exchange for allowing US oil companies to sell oil abroad after a decades-long export ban, the Republican-controlled Congress extended for their Democratic colleagues a set of tax breaks, which were set to expire at the end of 2016, that encouraged individuals and companies to invest in renewable energy.

Originally passed to limit climate change and rev the economy during the Great Recession, the tax breaks let homeowners and businesses write off a percentage of the cost of installing any number of renewable energy projects. The technologies included solar panels, wind turbines, geothermal pumps, fuel cells, and combined-heat-and-power units.

Or at least it did. In the rush to get home for the holidays, senators hashing out the details made a mistake, they said. Only tax breaks for solar and some wind energy were included in the final bill. Everything else was cut.

Lawmakers insisted to business groups and environmentalists that it was, in the words of Minority Leader Nancy Pelosi, a “drafting error.” Republicans assured Democrats that it would be quickly fixed.

But now, after a year of Washington inertia and a presidential election that has reshaped the political calculus of Congressional Republicans, that drafting error remains law.

With the tax breaks set to expire on Dec. 31, manufacturers employing the sorts of blue-collar workers that President-elect Donald Trump championed in the campaign are bracing for layoffs.

“It is mind-boggling,” Steve Smith, chief executive of geothermal pump manufacturer Enertech Global, which has a factory in South Dakota, told BuzzFeed News. “Even though I can’t see how it happened, I guess it apparently can happen.”

The drafting error appeared to occur on night of Dec. 15 last year during closed-door negotiations between Senate Majority Leader Mitch McConnell and Minority Leader Harry Reid, according to Bob Wyman, an independent energy consultant.

“The legislation was jointly prepared by staff from both parties and the exclusion of these provisions wasn't noticed until after the legislation was finalized,” Don Stewart, a spokesperson for McConnell, told BuzzFeed News.

Almost immediately, the error was found, stunning some legislators.

“I don’t have the details,” Republican Rep. Tom Reed of New York told BuzzFeed News recently. “Now people are obviously saying it was a staffing error. Obviously that may have occurred. But I’ve been around DC six years now, and to me, errors like that just don’t happen.”

The next day, Democratic Rep. Paul Tonko of New York, who has a fuel cell factory in his district and was one of the first to notice the mistake, proposed an amendment to add the tax breaks.

But House Republicans worried “changes at this time would threaten the entire package,” Republican Rep. Dan Newhouse of Washington said at the time. Without passing the end-of-bill omnibus bill, the government would be shut down right before Christmas. So Democrats, with a year to renew the renewable energy tax credits, backed down.

“We have commitments that will happen in an early revenue bill after the first of the year, and so we're pleased with that,” Pelosi said at a press conference two days later.

But when one of those first opportunities came in April with the reauthorization of the Federal Aviation Administration in the Senate, other senators began attaching their own amendments to the bill once the renewable energy tax breaks were added, toppling over the legislation.

A standalone bill introduced by Reed in May gained 40 co-sponsors, but similarly went nowhere.

A bigger hurdle had emerged by then, it turns out, to correcting the typo.

In April, more than 30 conservative groups, many funded by oil billionaires Charles and David Koch, wrote a letter to the Senate Finance Committee calling the renewable energy tax credits “a distortion of the tax laws for special interests in the renewable energy industry.”

With campaign fund donors’ stance against “green pork” clear, it became a tougher sell to the most conservative members of the House Republican Conference.

Still, Reid, McConnell, and Pelosi were "still on board to get this fixed in 2016," Doug Dougherty, head of the lobbying group Geothermal Exchange Organization, told BuzzFeed News.

Then, as with so many other political issues, everything changed with Donald Trump‘s victory.