Nine years later, during one of the longest economic expansions in American history, almost every Republican in Congress — including Mr. McConnell, now the majority leader — is set to vote for a tax plan that is projected to cause an even larger increase in the federal debt.

Republicans got to that extraordinary point by a tangled path. First they argued that the tax cuts would stimulate the economy enough to pay for themselves, only to be confronted on Thursday with a report that said the cuts would add $1 trillion to the deficit over a decade. That bad news led to last-minute convulsions and a frantic search for more revenue.

But in the end, it looked as though the political imperative to pass what would be the only major piece of legislation of President Trump’s first year, along with a near-religious Republican belief in tax cuts, would override their previous concerns about fiscal prudence.