President-elect Donald Trump has criticized Republicans who have voted to gut the independent body that investigates congressional misconduct.
Republicans voted in secret to close the Office of Congressional Ethics, against the advice of party leaders.
“Focus on tax reform, healthcare and so many other things of far greater importance!” Mr Trump said in a tweet.
Mr Trump made cleaning up corruption in Washington a key theme of his campaign.
He ended his tweet with “#DTS”, which is an acronym for “drain the swamp”, his campaign slogan.
Democrats reacted with outrage to the vote, which could be passed later in the first session of the new Congress.
Trump v Republicans – who wins? Anthony Zurcher, BBC News, Washington
Members of Congress have only just arrived back in Washington, and already there is a dispute brewing between president-elect Donald Trump and the Republican-controlled legislative branch.
The move on Monday night by House Republicans was already prompting condemnation by Democrats, good-government watchdogs and political commentators.
But given that Republicans will control all levers of power in the federal government and a public uproar seemed unlikely, the change appeared to be a fait accompli.
Then Donald Trump demonstrated just how quickly his itchy Twitter finger can alter the political dynamic. Because the president-elect weighed in against the move, Republicans in the House will now have to decide whether they want to cross their new standard bearer before he even takes the oath of office.
This has become an early test of Mr Trump’s power of political persuasion. Can he use the presidential bully pulpit, magnified through social media, to bend members of his own party to his will? Or will the man who campaigned on metaphorically draining the Washington swamp find his first steps mired in the muck?
Under the change:
The watchdog would no longer be independent
Lawmakers would vote to determine if a fellow member of congress has broken the law
The body would be prevented from receiving anonymous tips
Accusations against lawmakers would not be made public, as they are currently
Senior Republican Congressmen Paul Ryan and Kevin McCarthy had urged their party to seek bipartisan support and to wait to push for the change later.
But Virginia Congressman Bob Goodlatte submitted the proposal against the advice of his own party’s leaders.