Iowa’s two U.S. senators are touting the GOP’s tax plan, but it may not have enough support from their fellow Republicans to pass the senate. Senator Joni Ernst yesterday said she’s backing the bill because the economy has “under-achieved” during the current recovery.

“Last year the United States saw less than a two percent increase in the amount of goods and services we produced. And the reason? Our stagnant economy has suffered from an outdated tax system that stifles economic growth through high tax rates and an unreasonable compliance burden,” Ernst said during a brief speech on the senate floor.

Senator Chuck Grassley last week said the plan “makes good” on GOP tax-cutting promises.

“We now have a once-in-a generation opportunity to modernize our tax code in a way that will provide significant middle-income tax cuts while making the tax code simpler, fairer and more pro-growth,” Grassley said during a Senate Finance Committee meeting.

Wisconsin Senator Ron Johnson dealt a set-back to his fellow Republicans yesterday, though. Johnson announced he could not vote for the bill as currently written. Johnson said the bill unfairly tilts the tax relief to corporations. The GOP’s tax package now includes repeal of the federal fine for failing to have health insurance and a Republican senator from Maine is raising concerns about that, too. Susan Collins was one of three Republican senators who voted “no” earlier this year on a bill that would have repealed the entire federal health care law.

There are 52 Republicans in the U.S. Senate, so the GOP can afford to have no more than two vote against the bill.