LOS ANGELES — Starting in 2020, Los Angeles County’s 5.2 million voters will cast their ballots on new machines that the county had custom built over a decade to be highly accessible to citizens with all manner of disabilities and who speak 13 languages.

The new machines mark the biggest challenge in years to the highly consolidated voting machine industry in the United States, in which just three companies control more than 90 percent of the market. The dominant players have faced withering criticism from security advocates and lawmakers since the 2016 election for being too slow to adapt to election hacking threats from Russia and other adversaries and not transparent enough about their security.

The plan is for the machines to be tested at some voting locations during local elections in November and then to be used by all voters for the first time in primaries on March 3, 2020.

The challenge is even bigger because Los Angeles plans to make the computer code its machines are running on freely available to be used or modified by other voting jurisdictions.

But the new systems are also likely to add fire to a battle between cybersecurity hawks and advocates for voters with disabilities that’s already playing out in Congress and among state election boards.

The security hawks fear that Russia will take any opportunity to hack U.S. elections and want to eliminate technology from the voting process as much as possible. They say the majority of people should vote using hand-marked paper ballots — typically by filling out an oval similar to the ones that appear on a standardized test or else by drawing a line between an office and the voter’s preferred candidate for it.

Ballot-marking devices, or BMDs, like L.A. County’s should be reserved for people with disabilities who can’t mark ballots themselves, they say. One major concern is that even though those machines spit out a paper ballot that’s similar to a hand-marked one, voters are unlikely to check that the machine recorded everything accurately. Skeptics say that might give hackers a window to make unnoticed ballot changes that could tip a tight election.

In the case of both BMDs and hand-marked ballots, the totals are tabulated by machines. But jurisdictions typically verify that tally by hand-checking a small percentage of ballots.

Many advocates for hand-marked ballots also argue that BMDs are too expensive to be used by everyone.

“The experts are clear that hand-marked ballots are the safest way for most voters to vote,” said Sen. Ron Wyden (D-Ore.), one of the Senate’s main security hawks. “Ballot-marking devices can certainly play a role for voters with disabilities, but nationalizing the L.A. approach would be such a prohibitive expenditure it would not be on the table.”

Wyden is chief sponsor of the Protecting American Votes and Elections Act, the most far-reaching election security bill being considered in the Senate. It would mandate hand-marked paper ballots for most voters but allocate $250 million to buy secure BMDs for people with disabilities.

Most other election security bills sponsored by Democrats mandate paper ballots but don’t specify whether the voter must mark the ballot or if a machine can do it. Those measures are all highly unlikely to pass because they face fierce opposition from Senate Majority Leader Mitch McConnell (R-Ky.), who opposes any federal effort to mandate cybersecurity protections for state-run elections.

Disability advocates, however, say it’s unfair to have one system for voters with disabilities and another for voters without them. They also argue that reserving BMDs for people with disabilities can create other problems such as election workers who aren’t trained to operate the machines and a BMD market that’s so small that companies don’t have an incentive to build new and more secure machines.

They say any hacking dangers that BMDs create can be mitigated with other protections such as post-election audits.

“Those who are saying that BMDs should be limited to only people with disabilities, I think that’s a flawed premise from the start,” L.A. County Registrar Dean Logan said. “It creates a separate but equal type of scenario.”

Logan has led development of the L.A. County voting system since its inception in 2009, which included meetings with citizen advisory groups including people with disabilities, the elderly and those who spoke multiple languages.

The result looks similar to a massive iPad affixed to a plastic table with an oversized touch pad that voters use to enter votes and toggle between races. The iPad-like screen can be adjusted for people in wheelchairs and the touch pad buttons are big enough that voters with mobility issues can manipulate them with pointers and other tools. Voters can choose one of 13 languages and listen through earphones if they have impaired vision or prefer audio.

Logan also listed digital safeguards for the L.A. County machines — including numerous rounds of cybersecurity testing, procedures to ensure the machines are air-gapped from the Internet, and physical protections when the machines are stored and transported to and from polling locations.

This battle has played out mostly in places such as Georgia, which recently signed a statewide contract for ballot-marking devices that was marred by accusations of undue influence by voting machine companies. The contract came just six months after a hotly contested governor’s race between Democrat Stacey Abrams and Republican Brian Kemp, who was then the state’s top election official but refused to recuse himself from overseeing the vote.

There’s no controversy like that in Los Angeles, but the county’s sheer size is sure to lead to a debate about the future spread of BMDs.

L.A. County is by far the most populous local voting jurisdiction in the country and has more registered voters than 42 of the 50 states. The county routinely spends tens of millions of dollars on each national election and generates so many ballots on election night that it has to borrow six to eight Los Angeles County sheriff’s office helicopters to ferry them back and forth to its secure tabulating facility in Norwalk, Calif.

The system could spread even farther because the county plans to offer its software free to other jurisdictions. San Francisco, for example, is thinking about moving to an open-source voting system, but it hasn’t specifically focused on the L.A. system.

The L.A. system also could create an incentive for other jurisdictions to look at developing their own systems or put pressure on the major voting machine vendors — Election Systems & Software, Dominion Voting Systems and Hart InterCivic — to be more transparent about their security features.

Those companies have historically been hostile to letting outside researchers vet the cybersecurity of their software but have recently allowed some outside vetting by the Idaho National Laboratory.

“All eyes are on Los Angeles County,” said California Secretary of State Alex Padilla (D), whose office is in the process of certifying the L.A. system. “Local election officials I’ve come across all wish they had more and better options … [and] to the extent this serves as a motivator for legacy companies to up their game and offer newer, better, more secure voter products, I think it’s a great opportunity.”