Republican presidential candidate Mitt Romney is continuing to draw fire over his taxes.

Rivals Newt Gingrich and Rick Santorum are pressing the former Massachusetts governor to release his tax returns.

Wednesday, Romney revealed he pays a tax rate of 15 percent. That's about the same as what most middle-income Americans pay.

But since Romney is a multi-millionaire, liberals say he should pay more.

Romney says most of his income is from investments, which are taxed at the lower capital gains rate of 15 percent.

The Republican front runner says he'll release more details if he wins the Republican presidential nomination.