House Democrats set to retake the majority unveiled a political-reform bill Friday that would bolster ethics rules in Congress, push for automatic voter registration and require presidential nominees to release their tax returns — a direct rebuke of President Trump.

The package would require all political organizations to disclose their donors and create a national strategy to protect voting systems from outside interference and fund state upgrades, including the use of paper ballots to create a verified paper trail.

Responding to reports of Capitol Hill harassment in the “#MeToo” era, the bill would overhaul the federal ethics agency. It would also create a new ethical code for the Supreme Court, according to fact sheet distributed at a press conference.

Democrats said the proposals, which will be refined into bill text in the coming weeks, will appeal to Americans from coast to coast.

“These are transformative reforms that we’re putting forward,” said Rep. John Sarbanes, Maryland Democrat. “It’s a once-in-a-generation opportunity to make a bold proud declaration to the American people, ‘We get it, we hear you.’”

The package is part of the “For the People” platform that Democrat ran on this year. They flipped 40 seats to retake the majority for the first time in eight years.

House Minority Leader Nancy Pelosi said the public gave the new majority a mandate to focus on integrity in Washington and serve as a check on the other branches of government.

Democrats titled the bill “H.R. 1,” a symbolic tag that underscores its importance to the caucus, though does not mean it will be the first measure considered in the new Congress.

Last year, Republicans used the first-priority bill to revamp the tax code.

Democrats highlighted a section of the bill that would shore up the Voting Rights Act, after a 2013 Supreme Court ordered Congress to come up with a new formula regarding “preclearance,” in which states with a history of voting irregularities had to obtain federal approval for changes to their laws.

Mrs. Pelosi tasked Rep. Terri Sewell, a Democrat from Selma, Alabama, to lead the effort with Rep. John Lewis, Georgia Democrat and icon of the civil rights movement.

The blueprint also calls for an expansion of early voting and online registration, saying too many states are passing Byzantine laws that thwart would-be voters.

“It’s ridiculous that it’s an obstacle course to get to the ballot box,” Mr. Sarbanes said.

Requiring presidents to disclose their tax returns is a swipe at Mr. Trump, who broke with decades of tradition and refused to release the documents. He’s claimed they’re under audit and that people don’t care about them, anyway.

As it stands, Democrats could use arcane levers in the tax code to procure Mr. Trump’s returns through the Ways and Means Committee, though it’s unclear if they will take that step in the new year.

The Democrats’ bill would require candidates for president to release their past three years of returns within 30 days of securing the party nomination.

Democrats also plan to crack down on first-class travel by “public servants,” though sponsors are still drawing in the lines of who that covers.

Former members of Mr. Trump’s cabinet, Scott Pruitt and Tom Price, received a public spanking for their pricey travel before they were nudged out of office.

Another reform in the bill would seek to head off conflicts of interest by prohibiting members of Congress from serving on for-profit boards.

Rep. Chris Collins, New York Republican, was removed from the House Energy and Commerce Committee pending resolution of insider trading charges. Authorities allege Mr. Collins, who sat on the board of Innate Pharmaceuticals, instructed his son to sell his shares after he heard about a failed drug trial.

Mr. Collins said he expects to be exonerated. He won re-election earlier this month.