WASHINGTON—President Trump’s multitrillion-dollar economic agenda hinges on fractious GOP lawmakers who are at odds over the details of a tax package that could determine their fate in next year’s midterm elections.

Senator Ron Johnson of Wisconsin and Senator Rand Paul of Kentucky both indicated that they would seek major changes to the bill that passed the House.

President Donald Trump’s signature tax bill advanced from the House of Representatives early Thursday. Bond investors might not like it, but the bill is only going to get more expensive from here. The measure,