House GOP leaders face a bumpy road within their own party over the direction of a long-term transportation spending extension, even as they prepare for a vote Wednesday.

Although Speaker John Boehner (Ohio) remains hopeful his team can round up enough GOP support to pass a 90-day extension — a bill that would be used to start talks with the Senate on a longer extension — conservatives continue to chafe at the idea, worried it will mean the Senate’s version will prevail.

Specifically, conservatives worry that because the bill is a “clean” extension and does not include any of the reforms to highway and transit programs originally proposed by Boehner, they would automatically be precluded from being considered during a conference committee.

GOP aides said it was particularly frustrating to suddenly have conservatives both within the Conference and with outside organizations demanding reforms be included in the shell bill, given their vociferous opposition to Boehner’s original bill and the various iterations he has floated since February.

What was once seen as Boehner’s effort to leave his imprint on a fundamental aspect of the federal government has since devolved into a legislative nightmare. It was originally envisioned as a grand bargain of sorts linking revenues from energy exploration to spending on highway construction, but Boehner has been stymied by a coalition of conservatives and old-line Republicans with concerns over parochial issues ranging from port spending to union provisions.

Boehner has sought to sweeten the legislation with his latest offering by including language mandating the construction of the Keystone XL pipeline. But many conservatives have questioned the strength of the provision.

In a blog post Monday, Heritage Action for America bluntly warned against voting for the 90-day bill.

“Lawmakers and their constituents should understand that a vote for the 90-day extension enables the Senate-passed bill to become law. Wednesday’s vote is nothing more than a face-saving kabuki dance that ignores the ‘clear message’ sent by the American people just 17 months ago,” Heritage Action said.

Conservatives echoed those same concerns during a Members-only conference call on the bill organized by Majority Whip Kevin McCarthy (Calif.) late Friday afternoon.

During the call, several members of the Conference’s conservative wing pressed McCarthy on why Boehner was moving forward with a shell bill that does not include key reforms to the highway and transit programs.

According to one GOP source familiar with the call, Members repeatedly argued that “if this going to be our offer and our vehicle to get to conference, why don’t we put all the reforms into it?”

“Well, because it couldn’t pass the first time!” the frustrated source quipped, noting that conservatives have been one of the key stumbling blocks to Boehner’s months-long highway push.

McCarthy spent much of the weekend talking to individual Members to assuage their concerns. Although it is unclear whether he has the votes, the fact that leadership was pushing ahead with the plan could signal they are cautiously optimistic.

Still, Boehner is taking few chances. The Rules Committee today is expected to consider a closed rule for the bill, which would likely preclude conservatives from offering any amendments on the floor. Moving the bill under a rule also means Boehner needs only a bare majority. It is unclear whether Minority Whip Steny Hoyer (D-Md.) will whip against the bill, but it is unlikely Boehner can count on any Democratic support.

Boehner has also repeatedly said over the past several months that he does not want to pass the Senate’s bill, although he has not ruled it out specifically. Leadership aides said that while whatever the conference produces will not be to conservatives’ liking, at this point his maneuver might represent Republicans’ best option for shaping the final bill.

Still, conservatives remain skeptical and are pushing Boehner for more assurances.

For instance, on Monday Rep. Steve Pearce (R-N.M.) began circulating a letter to Boehner among members of the Republican Study Committee laying out conservatives’ demand for legislation devolving transportation programs to the states, while urging him not to bring the Senate bill or anything similar to the floor.

“House conservatives are determined to cut red tape and bureaucracy to make programs more efficient and effective. The American people deserve legislation that will get Washington out of the way and that helps improve our infrastructure by returning more revenue and authority to the states,” Pearce wrote in the letter.

“Ultimately, the Senate bill continues to send the current transportation system down an unsustainable path. In order to ensure that the system does not go bankrupt, House conservatives urge you to not take up” the Senate measure, he added. Pearce is expected to send the letter to Boehner later this week.