President Obama's re-election team is engaging conservative oil magnates Charles and David Koch in a public spat they hope will rouse the Democratic base and draw donations to their campaign.

The back-and-forth began Friday when, in a fund-raising email to supporters, Obama campaign manager Jim Messina accused the billionaire brothers of "jacking up prices at the pump" and bankrolling "Tea Party extremism."

Messina also said the Kochs have reportedly pledged $200 million, through the outside nonprofit group Americans for Prosperity, to help defeat Obama in November.

"Let's see how many of us can chip in $2 or more to the Two-Term Fund," he wrote.

The message drew a sharp response from Koch Companies spokesman Philip Ellender. In a public letter to Messina he denied the Kochs were trying to manipulate gas prices, and he said there was no hundred-million dollar pledge from them to target Obama.

"Americans for Prosperity is not simply 'funded by the Koch brothers,' as you state - rather it has tens of thousands of members and contributors from across the country and from all walks of life," Ellender wrote.

"It is an abuse of the President's position and does a disservice to our nation for the President and his campaign to criticize private citizens simply for the act of engaging in their constitutional right of free speech about important matters of public policy," Ellender said.

The group, which promotes lower taxes and fewer regulations for business, is registered as a nonprofit with the IRS to engage in issue advocacy. It does not have to disclose the identities or amounts of its donors, who can give unlimited sums.

But while Americans for Prosperity cannot engage in overt electioneering, it has unleashed a torrent of attacks on Obama over the past six months.  The group ran a $6 million ad campaign against the president leading up to the State of the Union, following a $2.4 million campaign last fall that focused on the Solyndra solar-power controversy.

Messina today challenged Ellender's defense of Americans for Prosperity, calling it a "cynical stretch" given its undisclosed sources of funding and negative ads.

"There is no campaign in the country that believes more in the active participation of Americans in the electoral process than this one," Messina wrote in a letter to Koch, first obtained by the Washington Post.

"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 Obama campaign has raised more than $118 million for the 2012 election, with nearly half coming from individuals giving in aggregate $200 or less, according to the Campaign Finance Institute.