WASHINGTON—House Democrats proposed extending the expanded child tax credit through 2025 and making permanent its key feature aimed at helping low-income families, as part of a plan to provide tax breaks for families and renewable-energy producers.

The plan, released Friday night, is the latest piece of Democratic legislation that could total $3.5 trillion over a decade. Lawmakers also aim to expand Medicare, create a national paid-leave program and attempt to address climate change, paid for with tax increases on high-income households and corporations.

Lawmakers are moving quickly, but they are also struggling to reach agreement on the size of the bill, the spending priorities and on what tax increases they are willing to accept. Democrats can enact the legislation without Republican support, but they can lose no more than three votes in the House and none in the Senate.

The Ways and Means Committee began considering its pieces of the bill this week and will continue work on it next week. Committee Chairman Richard Neal (D., Mass.) has been particularly circumspect about detailing proposed tax increases and Friday night’s 645-page proposal includes none of the major revenue-raising provisions that Democrats are expected to add later. Instead, it features a series of tax credits and incentives aimed at encouraging particular activities, often with complex eligibility rules.

“These proposals expand opportunity for the American people and support our efforts to build a healthier, more prosperous future,” Mr. Neal said.

All of this is subject to change as Democrats negotiate among themselves. House Democrats haven’t spelled out what tax increases they are willing to accept, with lawmakers divided over changes to international tax rules and capital-gains taxes. Democrats also haven’t said how they will change the cap on state and local deductions, a Republican policy stiffly opposed by Democratic lawmakers from New York and New Jersey. Senate Democrats have offered more tax increases, including recent plans to raise taxes on partnerships and stock buybacks.

The child-credit expansion is one piece of the Democratic agenda that will have the broadest impact on U.S. households, continuing the expansion that Congress and President Biden enacted in March.

Those changes increased the credit from $2,000 to $3,000, made the credit $3,600 for children under age 6 and allowed households to get the full credit even if they have little or no income. Congress also made the credit payable monthly, creating what is effectively a near-universal child benefit that Americans began receiving in July. The expanded portion of the credit begins phasing out once income reaches $75,000 for individuals and $150,000 for married couples.

Those changes expire after December. Many Democrats want to make those credit expansions permanent, but the cost would crowd other items out of their legislation.

Instead, they chose an extension that lines up with the 2025 expiration of major pieces of the 2017 tax law, meaning that Congress would consider all of those items together. Democrats could choose to shorten the extension, which would shrink the overall proposal or make room for other policies and then create pressure for Congress to approve another extension.

House Democrats did propose making one feature permanent—making the credit fully available to households regardless of income. Progressive advocates have pressed for that decision, arguing that making it permanent now would make it harder for Republicans to change later.

Mr. Neal’s proposal would extend a series of renewable-energy tax breaks through 2033, offering larger incentives for producers that pay higher wages and use apprenticeship programs. The legislation would extend tax credits for capturing and sequestering carbon dioxide and would extend incentives for biodiesel production through 2031.

Democrats would also offer at least a $7,500 tax credit to purchasers of plug-in electric vehicles, with larger benefits for vehicles manufactured domestically in factories with labor unions. Individuals making more than $400,000 and married couples making more than $800,000 would begin to lose the benefit and there are limits based on the vehicle’s recommended purchase price. The bill would create a separate new tax credit for purchasing used electric vehicles.

Victims of disasters since the beginning of 2018 would also get a benefit. Democrats propose retroactively repealing the limits on deductions for casualty losses. They would also add money to the low-income housing tax credit and the new markets tax credit, a program aimed at encouraging investments in struggling areas. The plan calls for creating a new tax credit for renovating homes in distressed neighborhoods.

Beyond the child tax credit, Democrats proposed continuing other changes from March’s coronavirus relief law. That measure expanded the earned-income tax credit and expanded tax credits for child care, which are now up to $8,000 for one child and $16,000 for two or more. They would also create a payroll tax credit for child care providers.

The legislation would empower the federal government to negotiate prices for at least 25 selected brand name drugs in 2025 that lack any competition, and at least 50 drugs in the following years until competitors enter the market. The upper price limit through negotiation could be no more than 1.2 times the average price of the drug in other countries.

Earlier this week, the Biden administration unveiled a plan to reduce prescription drug prices through a series of actions, including legislation.

—Stephanie Armour contributed to this article.

Write to Richard Rubin at richard.rubin@wsj.com