On Thursday, the government filed its brief to the Supreme Court in the case that will determine whether Obamacare subsidies disappear in three dozen states. Its argument is comprehensive, but one part of it speaks directly to the political history of the law, and the fact that everybody, including Republicans in Congress who now claim out of convenience that the law plainly limits subsidies to states that set up their own exchanges, always understood it to authorize subsidies everywhere.

The government confines this part of its argument to the legislative debate in the run up to the law’s passage in early 2010, but it could make the point more succinctly (and perhaps convincingly) by fast forwarding to early 2011. These days, Republicans up to and including Senate Majority Leader Mitch McConnell confidently pronounce that “the language of the law says … subsidies are only available for states that set up state exchanges.” But that’s not what they believed four years ago.

When Republicans took over the House in 2011, the political environment in Congress changed dramatically. Obamacare couldn’t be repealed, but it became fair game for damaging modifications, and the GOP took aim at it and other domestic spending programs whenever opportunities to offset the cost of new legislation arose. One of the first things Congress did back then was eliminate an Affordable Care Act provision that would have significantly expanded the number of expenses businesses are required to report to the IRS. Even before the law passed, business associations were livid about the “1099” requirement, and created such an uproar over it that the question quickly became how, not if, it would be repealed. Even Democrats wanted it gone.

The only problem was that the reporting requirement was expected to raise over $20 billion. Under GOP rule, it could only be offset with spending cuts elsewhere in the budget. As it happens, they found those spending cuts elsewhere in the ACA itself. Specifically, Republicans paid for repealing the 1099 provision by subjecting ACA beneficiaries to stricter rules regarding when they have to reimburse the government for subsidy overpayments. Make more money than you anticipated, and the government will claw back your premium assistance come tax season.

The congressional budget office scored the plan as essentially deficit neutral, and Republicans voted for it overwhelmingly. But you see the problem here. If the ACA plainly prohibits subsidies in states that didn’t set up their own exchanges, then there would be no subsidies in those states to claw back. And by April 2011, when the clawback passed, we already knew that multiple states were planning to protest ACA implementation and let the federal government set up their exchanges, including giant states like Florida, which now has a million beneficiaries. They would have needed a different, or additional, pay-for.