Sen. Marco Rubio (R-Fla.) could complicate things for the Republicans as they try to pass their tax overhaul, having told reporters on Thursday he won't support the legislation unless it increases the refundable portion of the child tax credit. Senate and House Republicans say they have reached an agreement on a tax bill, which lowers the corporate tax rate to 21 percent and top individual tax rate from 39.6 to 37 percent, and are just ironing out the final details.