Two of the Republican senators who have stepped forward to block the party’s dash toward a tax overhaul say they are withholding their support to stick up for Main Street. Big corporations, they argue, are being given preferential treatment compared with smaller businesses that are organized differently and known as pass-throughs.

The David-and-Goliath story that the senators, Ron Johnson of Wisconsin and Steve Daines of Montana, are spinning would be compelling — except that it is wrong.

Pass-throughs have a lower total tax burden than corporations do. That advantage exists now and remains in both the House and Senate versions of the tax bill.

If corporations are “the haves,” then pass-throughs are the “have-mores.”

“I honestly don’t know what he’s talking about,” Alan J. Auerbach, a tax expert at the University of California, Berkeley, said of Mr. Johnson’s complaints. “Tax rates are going down a lot. It’s businesses versus everybody else.”