WASHINGTON — No president has ever packed as much of his agenda, domestic and foreign, into a single piece of legislation as President Biden has with the $3.5 trillion spending plan that Democrats are trying to wrangle through Congress over the next six weeks.

The bill combines major initiatives on the economy, education, social welfare, climate change and foreign policy, funded in large part by an extensive rewrite of the tax code, which aims to bring in trillions from corporations and the rich. That stacking of priorities has raised the stakes for a president resting his ambitions on a bill that could fail over the smallest of intraparty disputes.

If successful, Mr. Biden’s far-reaching attempt could result in a presidency-defining victory that delivers on a decades-long campaign by Democrats to expand the federal government to combat social problems and spread the gains of a growing economy to workers, striking a fatal blow to the government-limiting philosophy of President Ronald Reagan that has largely defined American politics since the 1980s.

But as Democrats are increasingly seeing, the sheer weight of Mr. Biden’s progressive push could cause it to collapse, leaving the party empty-handed, with the president’s top priorities going unfulfilled. Some progressives fear a watered-down version of the bill could fail to deliver on the party’s promises and undermine its case for a more activist government. Some moderates worry that spending too much could cost Democrats, particularly those in more conservative districts, their seats in the 2022 midterm elections, erasing the party’s control of Congress.