This post has been updated
Keeping Wall Street banks in check and protecting everyday American consumers is an emerging core message of the Democratic Party moving toward the 2016 election. Warren's leading role in crafting the Consumer Financial Protection Bureau made her a star on the left and a leading voice in the party.
Asked repeatedly in an afternoon press conference whether she was prepared to block the spending bill on the Senate floor, Warren declined to say definitively, arguing the fight must first be taken up in the lower chamber, where the bill is expected to face a Thursday vote.
"Right now, the fight is in the House," said Warren. "That's the fight we are going to pursue. It is up to the House to strip this out. That's what keeps the government operating."
The spending bill would reverse requirements that banks "push out" some derivatives trading into separate, non-FDIC backed entities. The change has been sought by the banking industry.
"This is a democracy and the American people didn't elect us to stand up for Citigroup," Warren said on the Senate floor. She asked House Democrats to stand against the bill "until this risky giveaway is removed."
Both Democratic and Republican negotiators crafted the bill. But Warren rejected the idea that she was standing in the way of bipartisan accord.
"This isn't about compromise," she said. "It's about reckless behavior. It is about a giveaway to the largest financial institutions in the country. It is up to us to say no."
Rep. Maxine Waters (D-Calif.), who joined Warren, said there were "scores" of congressional Democrats opposing the bill right because of the banking regulation change. She said that conservative opposition to the bill gives House Democrats some sway.
"If they are expecting Democratic votes to put this out, then that gives us leverage. And I am hopeful we are able to use that leverage to pull this out," said Waters, the top-ranking Democrat on the Financial Services Committee.
House Republican leaders were confident Wednesday morning that they would be able to pass the spending bill Thursday. Some immigration hard-liners groused that the bill does not fight President Obama's executive actions on immigration hard enough. But a closed-door GOP meeting did not produce the same kind of anger past gatherings triggered.
Warren's concerns were echoed by some other Democrats, including House Minority Leader Nancy Pelosi (D-Calif.). While Pelosi did not say whether she would vote yes or no on the bill, she accused Republicans of trying to "stack the deck" in favor of banks.
"Buried in the more than 1,600 pages of the omnibus package Republicans posted in the dead of night are provisions to put hard-working taxpayers back on the hook for Wall Street’s riskiest behavior," Pelosi said.
In a written statement declaring himself a "no" vote, Rep. Chris Van Hollen (D-Md.) said: “It is unacceptable to threaten a government shutdown in order to do the bidding of the biggest banks and put taxpayers on the hook again for their gambling losses."
Rep. Jim McDermott (D-Wash.), who said he plans to vote no, called it "inconceivable" that Congress would undo a key part of Dodd-Frank.
"Why is Congress giving Wall Street a massive Christmas present, when so many hardworking Americans are struggling to make ends meet?" McDermott asked in a written statement.
Organized labor joined the call to remove the bank provision form the bill Wednesday afternoon. AFL-CIO President Richard Trumka issued a statement matching the concerns of Warren and others.