Former New York City mayor Rudy Giuliani (R) says he knows that lower taxes yield more revenue.

In his latest TV ad, Giuliani says Democratic presidential hopefuls Sen. Hillary Rodham Clinton (N.Y.), Sen. Barack ObamaBarack ObamaLetters: ATF should explain its ban on AR-15 ‘armor-piercing’ ammo Press: Hillary's doomed bid Overnight Energy: Trump to sign orders on offshore drilling, national monuments MORE (Ill.), and former Sen. John Edwards (N.C.), "are making the promise to raise taxes."

"The only thing I can tell you in addition to that is they'll raise taxes even more then they promise," Giuliani says.