WASHINGTON — In 2009, almost every Republican in Congress opposed a $787 billion stimulus plan in the midst of an economic crisis because they said it would cause a dangerous increase in the federal debt.

“Yesterday the Senate cast one of the most expensive votes in history,” Senator Mitch McConnell, the minority leader, said at the time. “Americans are wondering how we’re going to pay for all this.”

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 — supports 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.