Republicans in the largest conservative bloc in the House are leading mobilization efforts against the $1.2 trillion bipartisan infrastructure bill, which passed the Senate with GOP support in August.

The Republican Study Committee, chaired by Rep. Jim Banks of Indiana, has issued a series of warnings against the package since its introduction two months ago, referring to the legislation as the “Green New Deal Lite.”

Mr. Banks cited inflationary costs associated with the bill, in addition to the proposed government program expansions promised by the companion $3.5 trillion social safety-net spending package, which includes spending for tuition-free college and expanded health care, as “obvious” reasons why Republicans should object to both proposals.

“Part of our job at the Republican Study Committee is to plant the flag,” Mr. Banks said in an interview. “In this case, we planted the flag very early in that Republicans should oppose both these boondoggle spending plans that are linked.”

The RSC’s efforts come as House Democrats scramble to craft a compromise that would appeal to the liberal and moderate wings of their caucus on both bills, known as President Biden’s “Build Back Better” plan.

The conservative caucus, made up of more than 150 House Republicans, has helped influence GOP leadership to announce late last month that it will officially “whip” members to reject the $1.2 trillion infrastructure bill.

“It’s now clear that the tax-and-spend reconciliation bill and the Senate infrastructure bill are inextricably linked. Therefore, we will be whipping against both measures in an effort to stop Democrats from enacting $5 trillion in socialist spending,” said the office of House Minority Whip Steve Scalise, Louisiana Republican.

Jim Thurber, professor of government at American University, said the RSC’s size and scope give it leverage in the House, comparing it to the influence of the Congressional Progressive Caucus in the Democratic Party.

“I think it’s the most important legislative caucus,” Mr. Thurber said. “And when they’re against something, it’s significant.”

Mr. Thurber said the caucus, under Mr. Banks, has been able to modernize, emphasizing on digital communications and strategic messaging that’s given it increased authority in the GOP conference.

“I think the committee has become more modern than it was 20 years ago or something, and he’s part of that,” Mr. Thurber said. “As an individual, I think he’s highly respected.”

Under Mr. Banks’ leadership, the RSC initiates weekly newsletters, highlighting issues and conservatives’ stances on the majority Democrats’ agenda. Lately, the memos have targeted the negotiations over the spending bills, which have been the center of palace intrigue in Washington.

In one memo dubbed “Don’t fall for it,” the RSC warned its members against Democrats’ assertions that the reconciliation bill was decoupled from the infrastructure package.

“These aren’t two infrastructure packages. There’s just one being split in two for now, so that Democrats can pick up a few Republican votes to help them earn a ‘bipartisan’ label on the package,” the memo stated. “Once it passes, they’ll put the whole package together again and call the whole thing bipartisan.”

The RSC laid out a list of concerns about the bill on several messaging platforms, including the emphasis on instituting “equity” programs, an inconclusive cost estimate and economic impact report, and the lack of funding allocations going to traditional infrastructures like roads and bridges.

Rep. Kat Cammack, Florida Republican who is on the RSC’s steering committee, said asserting that the bills are linked is a key aspect of their message.

“We’ve known that the infrastructure bill and the budget reconciliation are linked — it’s a point Chairman Banks and RSC members have been discussing for nearly two months now,” Ms. Cammack said in a statement, adding that they’ll “continue to push back” as the largest conservative bloc in the House.

RSC member Rep. Kevin Hern of Oklahoma said he thinks more can be done to reach a compromise, despite 19 GOP senators, including Minority Leader Mitch McConnell, who ultimately supported the package.

“I know the senators, very moderate senators, worked hard to find [the common ground], but we can do a lot better with the American taxpayer dollars, and we’ve just got to stop this insatiable appetite to spend money that we don’t have,” Mr. Hern told The Washington Times.

Centrist Republicans in the House who previously signaled a green light for the infrastructure bill are also beginning to walk back their support, especially as the conference’s leadership warns against voting for it and the tension between Democratic progressives and moderates continues to play out.

Although at one point there were up to 20 House Republicans who would’ve been open to the bill, Mr. Thurber said that number is likely to continue shrinking as negotiations drag on and Republicans can hold more sway over their members.

“The longer they wait, the more difficult it will be to keep the Republicans that say they’re going to vote for it in the fold,” Mr. Thurber said.

In a private conversation with The Washington Times, a moderate Democrat said they were confident both bills will ultimately get passed, keeping in line with Mr. Biden’s agenda.

“It’s going to pass,” the lawmaker said. “Both infrastructure and Build Back Better are going to pass. No doubt about that.”

Rep. Jeff Duncan, South Carolina Republican, said the RSC is set to continue efforts to derail the bill and Mr. Biden’s agenda.

“The RSC was working hard behind the scenes to defeat the Biden agenda by calling out the infrastructure package for what it really is and coordinating strategy among Republican members,” Mr. Duncan said in a statement. “We must do all we can to combat the Biden administration’s socialist agenda.”

Asked if there were any benefits Mr. Banks sees from the infrastructure bill for his northeast Indiana district, which is in the heart of the Rust Belt, he said the negatives outweigh any advantages. His caucus has argued the measure largely benefits urban centers. 

“It’s rural districts like where I come from, and where many of my colleagues come from, that are the ones who are footing the bill,” Mr. Banks said. “I fail to see how any voter in my district would appreciate knowing that we would be raising their taxes, higher inflation, making it harder for them to make ends meet just because there were a few dollars to improve some roads and bridges.”