WASHINGTON — One of the new Republican Congress’s first legislative priorities — redefining a full-time worker under the Affordable Care Act — is gaining opposition just days before passage from a surprising group: conservatives.

The House will take up legislation on Wednesday, the first major bill of the 114th Congress, that would change the definition of a full-time worker under the health law from one who works 30 hours a week to one who works 40 hours. A vote is scheduled for Thursday.

Writing in early November in National Review, Yuval Levin, a conservative popular with House Republicans, said the legislation “seems likely to be worse than doing nothing.” His rationale is that there are many more people who work 40 hours a week than just over 30, and that it would be easier for an employer to cut their hours to 39 a week to avoid offering them insurance than to 29.

“Putting the cutoff for the employer mandate at 40 hours would likely put far, far more people at risk of having their hours cut than leaving it at 30 hours,” Levin wrote. “That would make for a worse effect on workers and on the economy. So by setting the definition lower, Obamacare’s architects were trying to mitigate the damaging effects of the employer mandate some, and by setting it higher Republicans would be worsening those effects.” Instead, he called for a full repeal of the employer coverage mandate.