The conservative Koch brothers issued an unusually harsh and public condemnation of a package of tax breaks hours before the measure was set for consideration on the House floor Tuesday, urging lawmakers to vote against the array of "corporate subsidies."

In a letter to lawmakers "on behalf of Koch Industries," Philip Ellender, president of government and public affairs at Koch Companies Public Sector, was especially critical of the Wind Production Tax Credit, which provides $9.5 billion a year in subsidies to producers of wind energy at the expense of traditional energy producers such as .... the Koch brothers.

"First enacted in 1992. the PTC's initial Intent was for the federal government to foster and support a nascent wind energy industry. But over time, like all subsidies and mandates, good intentions have led to harmful unintended consequences, turning a hand-up to an industry in its infäncy into a $23 dollar-per-megawatt-hour hand-out for big companies looking to pad their bottom-line," the letter says. "At a time when our nation is facing monumental economic challenges, this is the antithesis of the path we should be pursuing toward a simpler and less burdensome and wasteful tax code."

But it's not just the wind credit that's in the Kochs' crosshairs, the letter says. "We oppose ALL subsidies, whether existing or proposed, including programs that benefit us, which are principally those that are embedded in our economy, such as mandates."

Ellender concludes: "Koch will continue to lobby for the repeal of subsidies and mandates, as we work to make people's lives better. We believe history has proven that this is the best course to foster job creation, opportunity and prosperity."

While the Koch brothers and other conservative groups have long opposed the wind credit, Republican tax aides said this was the first time they could remember the Kochs launching such a public lobbying campaign.

Despite the Koch's enormous financial influence on Republican politics, a House GOP aides said they did not expect the letter to derail the measure scheduled for a vote late Tuesday afternoon. That bill would revive more than 50 expired tax breaks, including the wind credit, for the 2014 tax year and let them expire again on Dec. 31.

The measure would then go to the Senate, where Senate Finance Committee chairman Ron Wyden (D-Ore.) still has hope of crafting a longer-term bill that would preserve the breaks, worth roughly $45 billion to millions of companies and individuals, through the 2015 tax year.