Johnson. Photo: Tom Williams/CQ-Roll Call,Inc.

When Senate Republicans unveiled the latest version of their tax-cut plan last night, they claimed to have the votes in hand, and it appeared they might be correct. But now it seems they don’t. Senator Susan Collins said the higher premiums caused by eliminating the individual mandate will cancel out the benefits of lower taxes. “I have new statistics that show that for some middle-income people, it will cancel out their tax cut,” she said. “The increased premium would be more than the tax reduction they would get from this bill.”

Collins was already one of the most likely Republicans to defect. With 52 senators, they can only spare two votes. A much bigger problem arose when Senator Ron Johnson, an arch-conservative, came out in opposition today. Johnson complains that the bill’s treatment of pass-through businesses, which is quite generous, is not generous enough. “If they can pass it without me, let them,” he tells The Wall Street Journal. “I’m not going to vote for this tax package.”

It is hard to take Johnson’s threat to oppose the bill credibly, in part because he is so conservative, and in part because he made similar noises about Obamacare repeal before falling in line. But his opposition is bad news for Republicans.

First, if he forces the party leadership to give more generous treatment of pass-throughs, it would cost money. They don’t have much money to spare. Every business lobby in Washington has a hand out, and Republicans are already dealing with the headache that their bill raises taxes on millions of middle-class households.

Second, even if Johnson ultimately folds, merely forcing the party leadership to to negotiate with him harms the chances of passage. Republicans are racing to complete their bill, in part because the upcoming special election in Alabama might well deprive them of a Senate seat and a reliable vote. Democrat Doug Jones now stands a reasonably strong chance of defeating theocrat and notorious perv Roy Moore. Jones would be seated January 3, and would almost certainly vote no on any tax bill. The bill is polling in the mid-20s, and not even in Alabama is a yes vote going to help anybody.

Johnson is going to make party leaders spend more money or more time. And they don’t have much of either to spare.