Elections officials have tossed the voter registrations of George Mason University students who used a general campus address in their applications, prompting complaints from voting rights advocates.

The Fairfax County registrar rejected 171 registration applications last week, citing invalid residential addresses. The registrar, Gary Scott, said the students listed campus mailbox numbers and a general university address, making it impossible to know where they live and which county precinct, if any, they are eligible to vote in.

But lawyers with a national civil rights group accused Scott of running afoul of state regulations — and said his actions all but guarantee that most of the 171 students will be unable to vote in Virginia’s Nov. 5 elections.

The dispute highlights the difficulty college students often face when they try to vote while living away at school, with idiosyncratic campus mail systems often contributing to the confusion. Protecting that fast-growing slice of the electorate has become a major priority for voting rights groups, which are expanding their legal staffs and scouring the country for potential barriers to challenge as they prepare for the 2020 presidential election.

While most of the legal challenges are being made in Republican-controlled states, both Virginia’s and Fairfax County’s governments are controlled by Democrats.

“We take any policies or practices that might have the effect of suppressing student votes very seriously, and we’ll be working tirelessly to protect their rights in the 2020 election cycle,” said John Powers, a lawyer with the D.C.-based Lawyers’ Committee for Civil Rights Under Law, which has challenged the 171 application rejections in Virginia.

Scott, the Fairfax County registrar, said the origin of the problem is that GMU — a sprawling state university of more than 25,000 undergraduates about 20 miles west of Washington — allows any student to have a campus mailbox, whether they live on campus or not.

That made it impossible to discern eligibility for the students who listed their post box number with the school’s main campus address, rather than their dorm name or off-campus residential address, Scott said.

At least three of the 171 students actually live in neighboring Prince William County, meaning they’re not eligible to vote in Fairfax, he said.

“The point is, unless we know where on campus you reside, we can’t register you,” Scott said.

Powers said his primary objection is to Scott’s decision to send a “notice of rejection” to each of the 171 students. Virginia election law prohibits local registrars from rejecting an application because of insufficient address information. In that case, the registrar is supposed to send a request for information — not a notice of rejection.

Scott said that after mailing the rejection letters, his office sent follow-up letters to students specifying how to fix their application: by providing either a dorm name indicating that they live on campus, or a private residential address indicating they live elsewhere. The office initially gave students until Oct. 26 to act, then moved the deadline back to Nov. 2, three days before the election.

But Powers said the initial rejection letters could dissuade the students from fixing their registrations.

“To some extent the damage may have been done,” Powers said. “Some of these students may be first-time voters. Essentially what’s happened is, they’ve gotten a letter saying, ‘We rejected your application.’ There’s no mention of Oct. 26 or Nov. 2 or any kind of cure period.”

Scott conceded that students do not often check their campus mailboxes and may not have seen the correspondence. He said it’s too soon to say how many students will successfully fix their applications, as he just sent the most recent letter this week.

Carson Heisel won’t be one of them. Heisel, 18, a freshman studying forensic science at GMU, said he’s too busy with school to redo his application.

“I was definitely planning to vote, but I was not interested enough to go back and take the time and fill everything out,” he said.

If the students don’t fix their applications, they will be unable to vote in a momentous election on Nov. 5 in Virginia, the only state in the country where contests will determine the legislature’s balance of power this year.

Republicans are defending thin margins of 51 to 48 in the House and 20 to 19 in the Senate, with one vacancy in each chamber. With Democratic Gov. Ralph Northam in the Executive Mansion, a Democratic sweep of the legislature would give the party its first consolidated hold on power in a generation.

The Lawyers’ Committee is one of numerous national civil rights groups monitoring election practices across the country as the 2020 election approaches.

Kristen Clarke, the group’s president, said the goal is to increase the staff, now featuring about 35 attorneys, by about 50 percent in the coming months.

“There’s a larger threat, a greater need, more complaints to resolve and cases to be filed,” Clarke said. “We want to meet the demands that we face in this era.”

GMU is not the only university with addresses that can present complications to students seeking to register to vote. James Madison University, another state school in Harrisonburg, Va., is so large that it is divided into two precincts, meaning students must include their dormitory name when they register to vote.

In the city of Prairie View, Tex., which doesn’t have its own Zip code, students at Prairie View A&M University are sometimes assigned addresses listing the name of the neighboring town, prompting some of them to go to the wrong voting location.

One factor at GMU is the level of training of those working on registration drives. Maya Castillo with the New Virginia Majority, which conducted registration drives at GMU this year, said: “We’re generally aware of some of the issues around student voter registration. It’s something that we try as much as possible to train our team on. We want to make sure when we turn in voter registration applications they get registered. Sometimes that can get complicated.”

A GMU spokeswoman said the university has worked with the county registrar for years to “convey accurate information to students about voter registration.”

Rose B. Pascarell, the school’s vice president for university life, said once officials learned of the issues this year, they notified students to check their registration status.