Republican presidential nominee Mitt Romney began his first face-to-face debate with President Barack Obama by comparing the commander in chief to a boy who often said "something that isn't always true."

After Obama asserted that Romney's budget could not cut $5 trillion in taxes and raise military spending by $2 trillion and still expect to cut the deficit, the Republican candidate accused the president of not telling the truth.

"Virtually everything he said about my tax plan is inaccurate," the former Massachusetts governor quipped. "What I've said is I won't put in place a tax cut that adds to the deficit. That's part one. No economist can say Mitt Romney's plan adds $5 trillion."

"Number two, I won't reduce the share paid by high-income individuals. I know it's a popular thing to say with a lot of people, and it's not the case."

Romney added: "Look, I have five boys, I'm used to people saying something that isn't always true and keep on saying it hoping ultimately I will believe it."