But while few Democrats are willing to publicly say so, the details of the more than 800-page bill — which would radically reshape the way elections are run and make far-reaching changes to campaign finance laws and redistricting — have become a point of simmering contention. Some proponents argue that Democrats should break off a narrower bill dealing strictly with protecting voting rights to prevent the legislation, known as the For the People Act, from collapsing amid divisions over other issues…

A broad coalition of Democrats and liberal advocacy groups insist that the measure should not be broken apart, arguing that now is the time for an ambitious overhaul. But with Senator Joe Manchin III, a conservative West Virginia Democrat, opposed to the measure in its current form, Democratic leaders and Mr. Biden face tough decisions in the coming weeks about whether they can wrangle all their members behind it more or less as is, or must consider striking a narrower compromise…

Behind the scenes, two election lawyers close to the White House and congressional Democrats said Mr. Manchin was not the only one on their side with reservations about the measure. They insisted on anonymity to discuss the concerns because few Democrats want to concede that there are cracks in the coalition backing the measure or incur the wrath of the legion of liberal advocacy groups that have made its enactment their top priority.

Black House members, for instance, are deeply uneasy over the bill’s shift to independent redistricting commissions, which they fear could cost them seats if majority-minority districts are broken up, particularly in the South.