(Bloomberg) -- President Joe Biden is set to ratchet up pressure on congressional Democrats on Thursday, as discord within the party threatened to derail key pieces of his economic agenda, including lowering prescription drug prices and some of his proposed tax hikes on the wealthiest Americans.

In a speech Thursday at the White House, Biden plans to urge Democrats to unite, focusing on the struggles of middle class families -- such as the prices of prescription drugs, hearing aids and high-speed internet, as well as the more general costs of raising a child -- said a White House official. 

The recent back-and-forth on Capitol Hill has shown the limits of Biden’s power to guide the $3.5 trillion plan through a narrowly divided Congress. Moderate Democrats have balked at recent efforts to negotiate on drug prices and to impose a new tax on the assets of the wealthiest people, measures that progressives applaud.  

The White House can only afford to lose the support of three House Democrats for the package to pass; it needs all Democratic votes in the Senate. 

On Wednesday, Biden hosted two moderate Democratic senators -- Joe Manchin of West Virginia and Kyrsten Sinema of Arizona -- for separate meetings at the White House. Manchin has pushed for a spending package less than half of Biden’s proposal, a move that risks delaying the president’s package by weeks or months in Congress with tax, health care and other issues still unresolved. 

White House aides have spent the last several days negotiating and speaking with lawmakers and top staffers to weigh various options to raise enough money to fully fund Biden’s plan and ensure as many big policies as possible survive the legislative process.

Progressives have been disappointed as House Democratic lawmakers shied away from ideas like taxing the increase of wealthy people’s assets at death -- a key pillar of Biden’s attempts to reduce income inequality. Just this week, after intense lobbying from the health-care industry, a handful of moderate Democrats also backed away from efforts to negotiate lower prescription drug prices.

Democratic Representatives Kurt Schrader of Oregon, Scott Peters of California and Kathleen Rice of New York joined Republicans in voting against a bill to allow the government to demand lower prices from drugmakers and cap price increases on some medicines. The measure failed to advance, in a tied committee vote.

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