(Bloomberg) -- A House panel proposed far-reaching antitrust reforms to curb the power of U.S. technology giants including Amazon.com Inc. and Alphabet Inc.’s Google, culminating a 16-month investigation with a damning 449-page report that Republicans largely shunned.

The recommendations from the House antitrust subcommittee represent the most dramatic proposal to overhaul competition law in decades, and could lead to the breakup of tech companies if approved by Congress.

The findings target four of the biggest U.S. tech companies -- Amazon, Google, Facebook Inc., and Apple Inc. -- describing them as gatekeepers of the digital economy that can use their control over markets to pick winners and losers. The companies have abused their power to snuff out competitive threats, leading to less innovation, fewer choices for consumers and a hobbled democracy, the report said.

“Companies that once were scrappy, underdog startups that challenged the status quo have become the kinds of monopolies we last saw in the era of oil barons and railroad tycoons,” the panel’s Democratic leaders said. “These firms have too much power, and that power must be reined in and subject to appropriate oversight and enforcement. Our economy and democracy are at stake.”

Facebook fell more than 1% in late trading after the report’s release. Amazon and Apple slipped less than 1% and Google was unchanged.

The staff report’s most consequential recommendation is for Congress to consider legislation that would prevent tech companies from owning different lines of businesses, which could lead to a mandate to break them up.

“Their ability both to use their dominance in one market as negotiating leverage in another, and to subsidize entry to capture unrelated markets, have the effect of spreading concentration from one market into others, threatening greater and greater portions of the digital economy,” the report said.

To address this, the report recommends structural separation -- prohibiting a dominant platform from operating in competition with the firms dependent on it -- much like the Bank Holding Company Act of 1956 barred large banks from acquiring insurers, real estate firms, and other nonbanking companies.

It also calls for line-of-business restrictions, or limiting the markets in which a dominant firm can engage, similar to bans on television networks’ entering production and syndication markets.

Under congressional power, the breakups would target types of business, rather than particular companies, committee counsel told reporters on Tuesday.

“These ill-conceived ideas demonstrate a misunderstanding of the size and shape of the retail industry,” Amazon said in a blog post. “Amazon accounts for less than 1% of the $25 trillion global retail market and less than 4% of retail in the U.S. Unlike industries that are winner-take-all, retail has ample space for many winners.”

Facebook defended its acquisitions of WhatsApp and Instagram, which were criticized in the report as moves to eliminate nascent competitors and are under investigation by federal antitrust authorities.

“Instagram and WhatsApp have reached new heights of success because Facebook has invested billions in those businesses,” said a Facebook spokesperson. “Regulators thoroughly reviewed each deal and rightly did not see any reason to stop them at the time.”

Google said it has “invested billions of dollars in research and development to build and improve” free products like search, maps and Gmail. “We compete fairly in a fast-moving and highly competitive industry,” the company said.

Apple said it “vehemently” disagrees with the report and noted that it doesn’t have a dominant share in any market. The App Store developers “have been primary beneficiaries of this ecosystem,” said Apple, which defended its 30% commission rate as being “firmly in the mainstream” with rival stores.

The report is the culmination of an investigation announced by Cicilline last summer as federal antitrust enforcers were beginning probes of the dominant tech companies. The panel issued information requests that yielded millions of pages of documents and held seven hearings, including one in July that featured testimony by the chief executives of the four tech companies.

Republicans on the committee appeared poised to shun the recommendations from the Democrats, although Representative Ken Buck, a Republican member of the panel, told Bloomberg News on Tuesday some of his colleagues are interested in signing on to his response. A draft of Buck’s critique obtained Monday by Bloomberg News laid out areas of agreement and joint concerns about company behavior as well as proposals that are “non-starters” for conservatives.

Representative Jim Jordan of Ohio, who is the ranking Republican on the House Judiciary Committee, which includes the antitrust panel, criticized the report for what he called “radical” proposals to overhaul antitrust law.

“Big tech is out to get conservatives,” he said. “Unfortunately, the Democrats’ partisan report ignores this fundamental problem and potential solutions and instead advances radical proposals that would refashion antitrust law in the vision of the far left.”

Jordan authored a 28-page report focused on claims of anti-conservative bias by the companies. Five GOP members of the Judiciary Committee, including Buck, signed on. The top Republican on the subcommittee, Representative Jim Sensenbrenner, did not.

Business groups argued that Cicilline’s recommendations would harm consumers, punish success and overturn government’s traditional burden to prove its case. Groups advocating for more antitrust enforcement, meanwhile, argued it would reinvigorate consolidated markets and foster innovation.

The American Economic Liberties Project, which has called for the breakup of tech companies, praised it.

“Not only is this investigation and report a victory for workers, small businesses, and consumers, it is an immense achievement for the anti-monopoly and break up big tech movements,” said Sarah Miller, the organization’s executive director. “Congress should take up the subcommittee’s proposed remedies immediately.”

Other recommendations in the report include changing merger laws so that any acquisition by a dominant company is presumed to be anticompetitive unless the merging companies can show a takeover is “necessary for serving the public interest.”

Such a change would make it significantly easier for antitrust enforcers to stop deals by the big tech companies, which have completed hundreds of acquisitions over the past decade with little or no scrutiny. Dominant tech companies should also be required to notify enforcers of all transactions. Under current rules, many don’t have to be reported.

The subcommittee said it found that the dominant platforms have the ability to abuse their positions against suppliers, workers and consumers. But some of what the panel found objectionable wouldn’t fit the antitrust laws’ mold. To fix this, the report suggests creating a statutory red line, defining a seller as dominant if it has a market share of 30% or more and a buyer as dominant with just a 25% share.

The report also recommends increasing the ability of consumers to move their data between services, and for sites and services to work together more easily.

Companies such as Facebook are able to lock in consumers, the report contends, because few users would switch to an upstart that didn’t already have the powerful social connections that drive the companies’ networks.

The inability for consumers to switch between networks dims the possibility that competitors could dislodge Facebook. But if users could move their data, that might force Facebook or other companies to work with competitors, potentially cracking the company’s grip on consumers.

Congress has previously mandated portability standards when it forced telecommunications carriers to let consumers keep their phone numbers when switching networks. Phone dialing, and services such as email and texting, already work together across brands seamlessly.

Although Democrats control the House, Republicans’ longtime skepticism about changing antitrust law could dim the chances that Cicilline’s proposals are adopted. With time running out in this Congress, any real legislative action won’t happen until 2021. Even if the November presidential election hands the Senate majority to the Democrats, Republicans can still use procedural tools to block bills from passing.

Powerful business groups such as the U.S. Chamber of Commerce had already begun to push back on Cicilline’s ideas in the months leading up to the release of the report. They’re all but certain to ramp up their opposition with the unveiling of a bill.

“Any contemplated changes to our antitrust laws will impact all sectors of our economy,” the chamber’s president, Suzanne Clark, said. “We urge members of Congress to refrain from relying on this one-sided staff report to guide future legislation.”

The group launched a five-figure ad campaign this week to push a video defending the current laws, a spokeswoman said.

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