WASHINGTON — The Republican tax bill hurtling through Congress is increasingly tilting the United States tax code to benefit wealthy Americans, as party leaders race to shore up wavering lawmakers who are requesting more help for high-earning business owners.

On Monday, as Republican lawmakers returned to Washington determined to quickly pass their tax overhaul, senators were in feverish talks to resolve concerns that could bedevil the bill’s passage. With pressure increasing on Republicans to produce a legislative victory, lawmakers are contemplating changes that would exacerbate the tax bill’s divide between the rich and the middle class.

Those include efforts to further reward certain high-income business owners who are already receiving a tax break in the Senate bill but who are at the center of a concerted push by conservative lawmakers and trade groups to sweeten those benefits.

As Republican leaders pressed for a Senate floor vote this week, there appeared to be little momentum for amendments that would help low-income Americans, which some Republican and many Democratic senators had sought.