The threat was twofold. On the one hand, the grassroots and union leaders pledged to target lawmakers who back the Obama administration's wide-ranging trade push as it's currently constructed. On the other, the activists warned that Republicans will use the votes against Democrats.
"This is just a message to any member of Congress who is thinking about voting for this unfair trade deal," said Hasan Solomon of the International Association of Machinists and Aerospace Workers. "You need to start packing your bags right now, because we're going to fast-track you right out of office in 2016."
Solomon was referring specifically to fast-track legislation, known as Trade Promotion Authority, which would allow the administration to push looming trade agreements through Congress with no amendments on an expedited schedule.
Those deals, which are being negotiated in secret, include the Trans-Pacific Partnership with Pacific nations, the Transatlantic Trade and Investment Partnership with Europe and the Trade In Services Agreement that covers all service industries in the United States, Europe and two dozen other nations.
"Our members knocked on doors for you, they donated to you, they made phone calls for you, they are at your side," said Ben Wikler, the Washington director for MoveOn.org, speaking directly to Democrats. "But they are watching this vote, and they will not forget how you vote on fast-track."
Neil Sroka of Democracy for America laid out the problem Democrats could face from the GOP if they back a key part of the fast-track package. It's called the Trade Adjustment Assistance, and it pays to help retrain workers who are expected to lose their jobs because of the trade deals.
Democrats generally favor TAA, but the program would be funded in the current legislation by cutting some $700 million from Medicare.
Some House Democrats said Wednesday that Republicans were circulating a proposal to pass another bill later to undo the Medicare cuts, in hopes of swaying a few more Democrats. But Sroka predicted that even if the proposal is genuine, Republicans would turn around in 2016 and run ads accusing swing-state Democrats of cutting Medicare.
"Don't do it. It's a trap," Sroka said.
"Republicans and their billionaire backers are positively salivating at the opportunity to run attack ad after attack ad against you in the next election because you voted to cut Medicare benefits through the trade adjustment legislation," he added. "Even if it's going to be fixed, Republicans are going to attack you. And there's not a thing the White House, the Democratic Party, and certainly progressive organizations like Democracy for America ... is going to do to protect you."
The trade legislation could be voted on in the House as soon as this week if Obama and House Speaker John Boehner (R-Ohio) can find enough Democrats to get on board. Boehner is expecting to lose dozens of his Republicans, many of whom don't want to give Obama sweeping new powers.
Michael McAuliff covers Congress and politics for The Huffington Post. Talk to him on Facebook.