President Obama on Wednesday slammed opponent Mitt Romney and his running mate Rep. Paul Ryan (R-Wis.) for trying to sell a tax plan, which he dubbed “trickle-down snake oil.”

Addressing a crowd at a campaign event in Iowa on his final day of a three-day bus tour through the state, Obama elevated his rhetoric on his opponents’ tax plan, shifting slightly from the “trickle-down fairy dust” he called it a day earlier.

“They have tried to sell this trickle-down snake oil before,” Obama said at the outdoor event in Dubuque, where he was alongside first lady Michelle Obama in a rare joint appearance. “It didn’t work then; it won’t work now.”

Mitt Romney’s presidential campaign, anticipating harsh attacks over running mate Paul Ryan’s Medicare plan, has hit Obama hard for cutting $716 billion from the program, in his signature healthcare reform bill.