When President Donald Trump tweeted earlier this month about his apparent desire to help a Chinese telecommunications firm that skirted U.S. sanctions, Republicans on Capitol Hill largely brushed it off, confident that the president’s advisers would set him straight.

But when it became clear this week that his top negotiators for trade talks with China were prepared to go that route, congressional Republicans scrambled. Within hours, they had joined Democrats not only in writing letters to Trump and his cabinet officials—but also supporting, en masse, a legislative vehicle that wrests power away from the executive branch.

GOP lawmakers have grown visibly frustrated with the Trump administration amid high-stakes trade talks with Beijing—and they’re openly fearing that the negotiators will, against the advice of top intelligence officials, provide relief to China-based ZTE in order to reduce the trade deficit between the two countries.

“There’s just concerns about mixing national security with other things that don’t have to do with it,” Sen. Bob Corker (R-TN), a senior member of the Senate Banking Committee, said in an interview. “Then there’s the concerns about how the whole trade thing is being handled, period.”

Democrats and Republicans were alarmed when Trump tweeted earlier this month about his desire to save Chinese jobs by giving ZTE—the Chinese telecommunications giant that has evaded U.S. sanctions against North Korea and Iran, and which intelligence officials say poses national security risks to the U.S.—“a way to get back into business, fast.”

Republicans, while concerned with the president’s tweets, initially did not appear to put much stock in them. But as soon as they realized Trump might have been serious, senators took swift legislative action. The Wall Street Journal reported on Tuesday that the administration had agreed in principle to lift a U.S. Commerce Department ban that prevents ZTE from buying American parts—in exchange for paying a fine that, according to Trump, would exceed $1 billion. In a separate tweet this week, Trump said China would “purchase from our Great American Farmers practically as much as our Farmers can produce.”

The Senate Banking Committee on Tuesday, by a vote of 23 to 2, approved Sen. Chris Van Hollen’s (D-MD) amendment aimed at blocking the administration from unilaterally lifting existing penalties against ZTE without first certifying that the company is in compliance with U.S. laws. The existing ban has crippled ZTE, which relies heavily on American parts for its phones. American intelligence officials have warned that ZTE’s technology could be used to conduct cyber-espionage in the U.S.

In response to the reported deal between Washington and Beijing, Sen. Marco Rubio (R-FL) vowed “veto-proof congressional action” and accused the administration of “surrendering” to China.

“Those in the administration pushing for deal with #ZTE have access to same classified info we do about the national security danger posed by #ZTE & #China telecomm,” wrote Rubio, who spoke on the Senate floor for 25 minutes about the issue on Wednesday. “I don’t understand how they can push for a deal that lets them keep operating in U.S.”

Van Hollen, who authored the legislative push, told The Daily Beast that such a deal between the two countries “would send a terrible signal to anybody else who is currently violating U.S. sanctions or thinking about violating it, that if they catch you, you’ve just got to call up President Trump and he’ll give you a way out. That’ll unravel the entire sanctions regime.”

GOP lawmakers characterized the banking committee’s action as a pre-emptive measure to ensure that the president and his top negotiators would not compromise U.S. national security simply to win concessions from China on trade issues. The vote came just hours after Treasury Secretary Steven Mnuchin said the administration would not agree to a “quid pro quo” on ZTE and suggested that ZTE would be not used as a bargaining chip for trade talks. Senate Majority Whip John Cornyn (R-TX) described the entire ordeal as “confusing.”

“ I think we’re going to have to continue that conversation and make clear that from a ZTE standpoint it’s primarily a national security concern, and make sure that that’s appropriately addressed—because I’m not sure it is yet. ” — Senate Majority Whip John Cornyn

“I think we’re going to have to continue that conversation and make clear that from a ZTE standpoint it’s primarily a national security concern, and make sure that that’s appropriately addressed—because I’m not sure it is yet,” Cornyn said in an interview.

Cornyn circulated a bipartisan letter this week to Mnuchin, Commerce Secretary Wilbur Ross, and U.S. Trade Representative Robert Lighthizer, urging them to not “compromise lawful U.S. enforcement actions against serial and premeditated violators of U.S. law, such as ZTE.” The letter was co-signed by 26 other senators from across the ideological spectrum, and on Wednesday two key House members—Reps. Michael McCaul (R-TX) and Joaquin Castro (D-TX)—wrote a similar letter to the president.

The efforts were precipitated in part by lawmakers’ frustrations that they were being left in the dark on a high-stakes issue over which Congress has historically exercised broad authority.

“I think people just don’t know. I think people just want to understand—there’s been nothing said about what was on their mind when they decided to lift the sanctions,” a frustrated Corker told The Daily Beast. “It’s more of that, just what’s behind this and are we taking a national security interest and throwing it under the bus to deal with an issue that’s unrelated? People just want an understanding.”

Lawmakers are seeking to create a process whereby they can substantively push back against the Trump administration if negotiators agree to lift U.S. penalties against ZTE as part of a trade deal with China. Tuesday’s vote in the banking committee was the first step toward establishing that process, senators said.

“Congress ought to always utilize its powers particularly when it comes to trade, which are provided to us by the Constitution. We ought to be paying attention to what’s going on here,” Sen. Jerry Moran (R-KS), a member of the banking committee, told The Daily Beast. He said Congress has yet to gain an “understanding of what the administration’s strategy is.”

When Trump first announced in March that he was instituting a broad tariff policy targeting steel and aluminum imports, conservatives and Republicans on Capitol Hill lined up in opposition. They even warned that the risks of a trade war with other countries could cost American jobs and undermine the GOP’s efforts to highlight a strong economy in the run-up to the midterm elections. But they declined to respond legislatively, and instead vowed to work with the Trump administration to narrow the tariff policy.

But this time appears different.

“Republicans and Democrats alike think it’s outrageous that the president cut a deal with a Chinese company that flouts international laws and lies to the United States,” Sen. Sherrod Brown (D-OH), the top Democrat on the banking committee, said in an interview.

Sen. Mike Rounds (R-S.D.), who has also been outspoken on the issue, told The Daily Beast that senators began viewing it as a real possibility this week that the administration would give in to China on ZTE.

“We hope they don’t go that far,” Rounds said. “We just want to let them know how serious we think it is.”