My post colleague Aaron Blake has the scoop on a remarkable letter that the Obama campaign has just blasted in the direction of the billionaire Koch brothers.

The backstory: On Friday, Koch Companies — angered by an earlier Obama fundraising letter citing the Kochs — sent a letter to Obama’s advisers protesting that the Obama camp, by criticizing them, was squelching their right to grassroots political action.

Today’s response from Obama campaign manager Jim Messina fires back by ridiculing the idea that the Kochs are practicing grassroots politics:

You argue that Americans for Prosperity is a grassroots organization of everyday citizens. But its emphasis on rolling back environmental protections and blocking a clean energy economy appears to be nothing more than an effort to promote the corporate interests of your employers and others who lavishly, and secretly, fund its operations...

There is no campaign in the country that believes more in the active participation of Americans in the electoral process than this one. When you attempt to drown out their voices through unlimited, secret contributions to pursue a special interest agenda that conflicts with what’s best for our nation, you must expect some scrutiny of your actions.

The Koch brothers have been stirring up the Dem base ever since the Koch-founded Americans for Prosperity’s mobilizing of the Tea Partrty and its spending in the Wisconsin recall battles. Keep in mind that the Obama campaign’s first ad directly targeted (without naming) them for their role in bankrolling attacks on Obam’s energy policies.

For the Obama camp, this is a twofer. On the one hand, turning the Kochs into the walking embodiment of the secret money sloshing into our politics helps personalize an issue — campaign finance — that risks remaining abstract for many voters. But more important, this is also all about pushing back on GOP attacks over gas prices.

The singling out of secretive oil billionaires could help divert public anger over an issue that Republicans are vowing to make central to the presidential campaign. This is emerging as a concerted Dem strategy: DCCC chair Steve Israel told me the other day that House Dem candidates are being urged to fight back on gas prices by highlighting the House GOP’s support for oil company subsidies.

Republicans will likely argue that this feeds into a story they like to tell about Obama’s alleged use of bigfoot government power to bully private citizens. On the other hand, that could put Republicans on the side of oil billionaires at a time when Obama is proposing corporate tax reform that would roll back Big Oil subsidies that Republicans are defending.

Either way, the Obama campaign clearly sees advantage in sustained engagement with the Koch brothers — it’s another way of framing the larger arguments of Campaign 2012 — so expect more of this.