WASHINGTON (AP) — The Republicans' razor-thin margin for driving their sweeping tax package through the Senate was thrown into jeopardy Thursday when GOP Sen. Marco Rubio declared he will vote against it unless negotiators expand the tax credit that low-income Americans can claim for their children.

Rubio's potential defection complicates Republican leaders' goal of muscling the $1.5 trillion bill through Congress next week, handing President Donald Trump his first major legislative victory by Christmas.

Senate Republicans could still pass the package without Rubio's vote, but they would be cu...