Conservative political activists are warning congressional Republicans tempted to cave on the 20 percent corporate tax rate included in both the Senate and House versions of the bill — 20 percent is the compromise figure.

Five leaders in the tax-reform movement told reporters on a conference call Monday that retreating even by a percentage point or two would be more deleterious than it might seem. Grover Norquist, president of Americans for Tax Reform, said President Donald Trump initially wanted to slash the top corporate rate from 35 percent all the way down to 15 percent.

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“So 20 was a compromise,” Norquist said. “We wanted to be at 15 … If we want to really be competitive, we need to be at 15 because a lot of state and local governments have income taxes. Fifteen is 20; 20 is 25. So, if you move from 20 up to 21 or 22, this is not a resting point.”

After that, Norquist said, it will become easier for politicians to justify additional upward adjustments.

Norquist and the other leaders who addressed reporters are representatives of 27 organizations — “a virtual who’s who of the limited-government movement,” according to National Taxpayers Union president Pete Sepp — who signed a letter last week urging the House-Senate conference committee to leave the tax legislation’s 20 percent rate unchanged.

Norquist and Sepp were responding to media reports that some Republican lawmakers are considering reducing the tax cut. That would put the top rate close to the 22.5 percent global average and undermine the economic benefits of the legislation, speakers said.

“The House and Senate did the right thing when they both passed a bill that kept this very important rate,” said Christine Harbin, vice president of external affairs for Americans for Prosperity. “This will bring economic growth and wage growth on top of the middle-class relief they’ll see on the other side, on the personal side.”

Harbin, however, stopped short of threatening to oppose the bill if the corporate rate cut is slightly less generous.

Sepp noted the Congressional Budget Office estimates 25 percent of the tax relief to corporations will be channeled into increased wages. Other economists project that it will be as high as 50 percent.

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Sepp said Americans spend $163 billion a year to comply with the complex tax code. About two-thirds of the costs are due to business taxes, he said.

Andy Roth, vice president of the Club for Growth, blamed special interests for lawmakers’ waffling on the 20 percent rate. “The sad fact is that politicians are trying to save their little carve-out, and their little exemption that they want to keep in,” he said.

“While neither bill is perfect, reducing the corporate tax rate from 35 to 20 would send a message to the rest of the world that America is going to compete.”

Roth urged Congress to maximize simplicity and growth. “Just go with the most pro-growth tax cut that both chambers and the White House have agreed to,” he said.

Nathan Nascimento, executive vice president of Freedom Partners, told reporters that Congress should strive for a corporate tax code featuring low rates that apply evenly across industries and individual companies.

“While neither bill is perfect, reducing the corporate tax rate from 35 to 20 would send a message to the rest of the world that America is going to compete,” he said. “And importantly, it would begin to level the playing field for all businesses, especially those that don’t get the special carve-outs in today’s tax code.”

During the Senate debate over the Tax Cuts and Jobs Act, a majority rebuffed a proposal by Sens. Marco Rubio (R-Fla.) and Mike Lee (R-Utah) to pare back the corporate tax cut to make way for letting lower-income Americans claim the full value of the child tax credit by deducting it from their payroll taxes.

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Rubio argued that reducing the cut by a percentage point would do nothing to impede the growth benefits.

But Norquist told reporters Monday it would repeat the mistake of past presidents of both parties who let the corporate rate rise in small increments.

“What Rubio is doing was channeling his inner George Herbert Walker Bush, who said, ‘No one will mind it going up to 28, will they?’ … All marginal tax increases cause problems,” he said. “When you open the door to raising the rate, you’re not the only guy who gets to walk through. There will be future Congresses and senators and presidents who will be emboldened to take that rate up, using the exact same rhetoric that George Herbert Walker Bush Rubio used.”

PoliZette senior writer Brendan Kirby can be reached at [email protected]. Follow him on Twitter.