Sen. Tom Coburn’s war on ethanol tax subsidies — and his ideological battle with anti-tax crusader Grover Norquist — will finally come to a vote Tuesday after the Oklahoma Republican launched a sneak attack that caught opponents and party leaders off guard.

Last week, Coburn effectively hijacked the floor, using Senate rules in a surprising way to force the issue.

Earlier in the week, he had introduced an unrelated amendment to the Economic Development Administration bill. On Thursday, he called it up, substituted it with one gutting ethanol tax subsidies and tariffs, and immediately filed a motion to cut off debate before anyone knew what was happening.

Any Senator can demand a vote to end debate —or file a motion to invoke cloture — with the support of 16 Senators, which Coburn had quietly secured.

The move is extremely unusual in the Senate, where it is generally the prerogative of the Majority Leader to set the schedule. But it’s a sign of the frustration Senators have with a sclerotic chamber where both parties routinely block votes and little is getting accomplished.

Coburn’s gambit had Norquist’s group, Americans for Tax Reform, scrambling to play defense. Norquist considers Coburn’s amendment a tax increase and a violation of the pledge most Republicans have signed to never support one.

The dispute over the $6 billion a year ethanol tax credit is also a proxy fight in the larger war over whether Republicans should support eliminating tax breaks as part of a broader deficit reduction package.

“The days of placing spending programs in the tax code and giving them holy status are over,” Coburn said in a statement Friday.

Norquist said Coburn is on a mission to legitimize tax increases.

Coburn is trying “to get a bunch of Republicans to vote for a small tax increase, and then he says, ‘A ha ha!’ Then, having established this principle, let’s go ahead and raise a trillion in taxes,” Norquist said Friday.

“He may win some votes on this ethanol thing … [but] I think when he tries to turn that into a trillion-dollar tax increase, he’ll find he doesn’t have as many friends as he thought,” he added.

Norquist also favors eliminating the ethanol credit, but only if it is offset with a tax cut somewhere else.

However, conservative momentum for the amendment has been building, a scenario that threatens to hand Norquist a mass repudiation of his tax purity test. So he looked for a way to let pledge-signers vote for it and still stay kosher.

Enter Sen. Jim DeMint. The conservative South Carolina Republican strongly endorsed Coburn’s amendment Friday and announced he will offer a separate amendment to eliminate ethanol mandates as well as the estate tax. Norquist’s group gave its blessing for pledge-signers to back Coburn’s amendment, so long as they also vote for DeMint’s. But that’s problematic — there’s no guarantee DeMint’s amendment will receive a vote, and it’s very likely to fail even if it does.

According to the group, a vote for a final bill that includes the Coburn amendment, but not the DeMint amendment, would still violate the pledge.

Norquist’s ploy could have the effect of muddying the water for the Tuesday vote. But the momentum had clearly swung to Coburn in recent days. Coburn has marshaled strong backing from conservatives, including the Club for Growth, which called the ethanol subsidy “an abomination” and announced Friday that it will log the vote on its influential annual scorecard. Taxpayers for Common Sense also issued a report arguing the ethanol industry should finally stand on its own.

Of course, an initial victory doesn’t mean Coburn will succeed in killing the credits. The underlying bill may collapse. Even if it gets to the House, it faces an uncertain path to President Barack Obama’s desk. And it’s not clear if Obama would sign it, given the importance of Corn Belt states in the 2012 elections.

But a strong vote for Coburn would be a big setback to the ethanol industry’s attempt to extend at least some of the 45-cents-per-gallon tax credit beyond the end of this year.

The industry, which has been blanketing Capitol Hill with advertisements touting the benefits of the homegrown fuel, immediately accused Coburn of an “ambush” that would, if successful, help the likes of Libya and Venezuela by increasing the United States’ reliance on foreign oil.

Jill Kozeny, a spokeswoman for Sen. Chuck Grassley (R-Iowa), the leading defender of ethanol, ripped the amendment as “out of touch” — a job killer that would raise gas prices during a tough economy when drivers are already getting pinched at the pump.

Other ethanol backers were preparing amendments that would give Senators an alternative to support.

Opponents — such as meat producers hoping for cheaper feed prices — likewise ramped up their efforts to kill the subsidies once and for all. In addition to the cost to taxpayers, they argue, ethanol drives up the cost of food and lowers gas mileage.