The one thing that could still torpedo Trump’s big beautiful bill.

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President Donald Trump’s “one big, beautiful bill” is heading to the Senate. It would revise the tax code, cut Medicaid rolls, roll back clean energy programs, and increase funding for border enforcement. After weeks of wrangling, the House passed the bill last week by the thinnest of margins. The bill’s fate, though, ultimately depends on an obscure Senate rule: the Byrd rule.

Republicans are trying to pass the bill through the budget reconciliation process, which allows the Senate to close debate with a simple-majority vote—an exception to the usual 60-vote filibuster threshold. The problem for Republicans is the Byrd rule, which provides that provisions can be