A Senate Republican tax plan that would repeal the Obamacare mandate and give permanent tax cuts only to U.S. corporations drew fire from two Republican lawmakers on Wednesday in what could be a sign of trouble for the sweeping measure.

Republican Sen. Ron Johnson of Wisconsin said he would not vote for the proposal being debated by the Senate Finance Committee, telling the Wall Street Journal that it unfairly benefited corporations over other kinds of businesses.

Sen. Susan Collins, one of three Republicans who opposed a Republican Obamacare repeal effort earlier this year, also warned that some middle-income taxpayers could see tax cuts wiped out by higher health insurance premiums if the repeal of the Affordable Care Act's mandate goes through.