President Barack Obama's re-election campaign on Tuesday issued a memo knocking the economic plans of Republican presidential frontrunners Rick Santorum and Mitt Romney.

The memo, addressed to interested parties and sent to the press, represents the latest sign that the Obama campaign now considers Santorum a potential competitor for the president in the fall.

Until Santorum's recent rise, national Democrats have almost solely trained their fire on Romney. But with Santorum surging ahead in the GOP horserace and now leading Romney in national polls, the Obama campaign appears to be shifting gears.

In this instance, the memo goes in depth on the economic plans presented by both Romney and Santorum, suggesting the campaign sees little difference between the two candidates

The campaign paints the plans, both of which focus on lower taxes and lower spending, as unrealistic.

Santorum has proposed cutting $5 trillion in federal spending in five years with the goal of reaching a balanced budget in the fifth year

Team Obama, however, said the numbers don't add up.

"Even if Santorum achieves nearly a trillion dollars a year in savings, nearly all of those savings would be needed to finance Santorum's tax cuts, leaving little or nothing for deficit reduction," James Kvaal, the campaign's policy director, said in the memo.

Citing the Tax Policy Center, the lengthy document claimed Santorum's tax plan, which includes a 17% corporate tax and zero tax rate on manufacturers, would raise the deficit by about $900 billion in 2015.

As for individual tax rates, Santorum would reduce the number of income tax brackets from six to two (10% and 28%) and triple what his campaign identifies as the personal deduction that parents can claim for their children. In addition, he would eliminate both the Alternative Minimum Tax and the estate tax. And he would reduce the capital gains rate from 15% to 12%.

Romney's plan would get rid of taxes on interest, dividends and capital gains for taxpayers who make less than $200,000. It also calls for an elimination of the estate tax, and a reduction in the tax rate paid by corporations from 35% to 25%.

But Obama's team said both plans would shift the tax burden to the middle class.

"Like their budgeting, the rest of Romney and Santorum's economic policies return to the trickle-down, boom-and-bust economics that failed in the past," Kvaal said.