The GOP tax bill has cleared another hurdle, with approval by a party-line vote of 12-11 in the Senate Budget Committee Tuesday afternoon, thehill.com reports.  Two Republican holdouts voted "for the measure after previously expressing concerns":

Monday evening, two GOP senators, Ron Johnson (Wis.) and Bob Corker (Tenn.), had left the door open to voting against the measure in committee if their concerns weren't addressed before then.  Johnson has expressed concerns that the bill doesn't do enough to help pass-through businesses, while Corker is concerned about the impact on the debt.

Corker, along with Oklahoma senator James Lankford and Arizona's Jeff Flake, has been "working with leadership on a 'backstop' in case tax receipts crash," and after the vote, Corker confirmed he has an "outline of an agreement" on the backstop.

The trigger for the backstop is unclear, but someone's taxes would go up under certain conditions, creating "uncertainty" about future tax rates.  Only Congress could make scrambled eggs out of a simple tax cut. 

Senator Johnson, along with Montana senator Steve Daines, has been negotiating with leadership for further tax reductions on pass-through businesses, where profits are taxed at the individual rates.  Johnson was willing to advance the bill to the full Senate, a good sign for ultimately getting his vote.

Johnson's concern is that small businesses will be taxed at a much higher rate than corporations, as former Trump economic adviser Steve Moore spells out at townhall.com:

Sen. Johnson's beef with the House plan is that the effective tax rate on a successful small business would be almost 36 percent (as opposed to 39.6 percent today), compared with the new 20 percent rate for a corporation, such as General Electric Co. or Boeing.

While other factors affect overall rates, Moore agrees with Senator Johnson that the GOP tax plan should "provide more relief for small businesses, which create more than half the jobs in our country": 

Another idea is to increase the exclusion on small-business income from 17 percent (in the current Senate plan) to closer to 20 percent. This would get the effective rate down to just above 30 percent for family-owned businesses.

Moore also proposes on offset to the increased small business exclusion to avoid "breaking the budget rules":

Eliminate the deductibility of state and local taxes for corporations. It is wildly unfair for a small business to lose this deduction while corporations keep it. The strong case for elimination of state and local tax deductibility equally applies to families, small businesses and large corporations. Tax filers located in low-tax jurisdictions should never pay more tax to subsidize those located in high-tax locales.

As Moore observes, eliminating the deduction for corporate state and local taxes would be a "two-for-one policy victory," providing funds for small business tax cuts and equalizing tax treatment of small and large businesses, while also blunting the Democrat talking point of tax cuts for big corporations.  Johnson and Daines are said to be on board with the Moore proposal.

Adding the corporate deduction repeal might, however, also "engender still more resistance" from blue-state House Republicans who are already balking at repeal of the individual deduction for state and local taxes.

The threat of repealing the individual deduction for state and local taxes is already "setting off budgetary alarm bells in high-tax states like New York, California and New Jersey," as politico.com reported last week.  Repealing the corporate deduction would put blue-state politicians on notice that their big spending and unfavorable business climate would no longer be subsidized by the rest of the country. 

While Kentucky senator Rand Paul now says he will vote for the tax bill, several other senators remain on the fence: 

• Alaska's Lisa Murkowski now says she supports the Obamacare individual mandate repeal in the Senate tax bill, as long as the Obamacare cost-sharing subsidy bill, which President Trump told Republican senators on Tuesday he will support, is passed at the same time.  But based on her track record, Murkowski's vote is "not guaranteed."

• Maine's Susan Collins still has issues with the individual mandate repeal, as well as with the temporary nature of the individual tax cuts and the one-point reduction in the top rate.  Eliminating the mandate provides the GOP with "more than $300 billion" to cover other aspects of the tax cuts, making it hard to drop that provision.  As Bloomberg.com notes, Collins is "from a reliably Democratic-leaning state, and as such she's always a difficult vote for party leaders."

• This leaves Arizona's John McCain, who infamously deep-sixed the last attempt at Obamacare repeal, telling the president in a last-minute phone call at the time, "This is insanity," as nypost.com reports.  McCain has emphasized  regular order and bipartisanship and is also focused on any effects on military spending.  He remains a wild card in the process.

With the tax bill going to the Senate floor for a possible vote later this week, the GOP can afford the defection of only two of the 52 Republican senators, with Vice President Mike Pence casting the deciding vote to pass the bill under budget reconciliation rules.  Senators Corker, Lankford, and Flake, the budget hawks, are working with leadership on a path to a vote, and Johnson and Daines are negotiating a small business fix.  Murkowski may have a path between the mandate repeal and the insurer subsidy, while Collins and McCain look less certain so far.

With the Republicans racing against the calendar to get a tax bill to the president's desk before the Christmas break and before a possible Democrat is seated from the Alabama election, the GOP tax bill has cleared another hurdle, with several more to go.