WASHINGTON — In a 1,600-page, $1.1 trillion spending bill, a provision to roll back an obscure financial regulation became a focal point of uproar as Congress struggled to keep the government funded.

The “push-out” regulation — a measure to ensure that banks trade their riskiest financial instruments without the protection of the Federal Deposit Insurance Corporation or the Federal Reserve’s backup — was controversial from the start. Hundreds of billions of taxpayer dollars were shoveled into Wall Street banks after instruments like credit default swaps became worthless in the financial crisis, but even some crucial Democrats were unsure if Congress went too far when it voted to include push-out in the landmark Dodd-Frank law to regulate Wall Street in 2010.

But with regulators pressing to put rules into effect to carry out the law, a provision in the enormous spending bill to remove the push-out regulation drew bipartisan outrage. Representative Nancy Pelosi of California, the House minority leader, said she was “heartbroken” by the “taint” visited upon the spending bill, which would finance virtually all of the government through September.

Senator David Vitter, Republican of Louisiana, one of the Senate’s most conservative lawmakers, teamed with Senator Sherrod Brown, Democrat of Ohio, one of its most liberal ones, to demand the provision’s removal.