The effort comes as Republicans, who won a historically small House majority last week, toil to overcome their divisions and figure out how to govern. Democrats are primed to do as much as they can to disrupt the delicate balance that Mr. McCarthy will have to strike, should he win the speakership, to keep a handle on his fractious ranks.
The new liberal groups intend to work in cooperation with other, more established organizations that are also gearing up for showdowns with the new House majority and believe their experience in monitoring Republicans in recent years will prove an advantage. At the same time, the Biden administration and Democratic lawmakers are preparing for the onslaught, bringing on staff experts, communications specialists and lawyers versed in investigations to help with responding to the Republican efforts.
“We are more ready than I feel we have been before,” said Lisa Gilbert, the executive vice president of the watchdog group Public Citizen, part of an existing coalition called Not Above the Law, which has focused on what it sees as abuse of power by former President Donald J. Trump and his allies. “We know the playbook in a real way.”
Mr. McCarthy’s office did not respond to a request for comment on the emerging campaign. But Republicans are aware that well-funded groups on the left will do what they can to stir opposition to the new majority.
In an appearance Sunday on Fox News, Mr. McCarthy made it clear that he would not be deterred from leading an ambitious House majority if he can secure the votes to become speaker, a tall order given opposition to him on the far right and the party’s razor-thin vote margin. He argued that fellow House Republicans should rally around him so they could follow through with their legislative proposals and plans to shine a spotlight on the Biden administration.
“All the work that we want to do — from rolling out the investigations, from rebuilding our economy, from securing our border — none of that can go forward without us moving forward on the floor, having the vote of the speaker, putting the rules in and working together as one,” Mr. McCarthy said.
While those on the left have been organizing, doing research and mapping plans for serving as the opposition for months, the surprisingly close battle for the majority and the extended uncertainty about which party would prevail gave rise to a brief glimmer of optimism among their allies that Democrats might hold the House and they could shelve their work. But with the issue settled, they say they are now ready to move forward.