But tribal officials and Democrats say it appears aimed at making it harder for thousands of Native Americans to vote, particularly those who live on reservations without conventional street names. The law specifically bans the use of P.O. boxes as an alternative form of address, rendering many tribal ID cards invalid.

Native American activists have responded with plans to create addresses on the spot for those who need them on Election Day.

Tribal officials will stand outside polling stations on Nov. 6 with laptops and access to rural addressing software and a shared database of voter names. North Dakota is the only state that does not require voter registration, meaning eligible voters can generally show up at the polls and cast a ballot so long as they have proper identification.

O.J. Semans, chief executive of Four Directions, a national Native American voting rights group, said the strategy was “legally watertight” and necessary to counter the “devastating” court ruling.

“Even if it doesn’t change the overall result, it’s about fighting back,” Semans said. “We have to fight back.”

In one of the country’s least-populous states — and where Heitkamp, one of the Senate’s most endangered Democratic incumbents, eked out a victory of fewer than 3,000 votes in 2012 — the Supreme Court ruling could prove decisive.

Native Americans were widely credited with delivering Heitkamp’s last win, which set in motion a six-year legal war of attrition pitting the GOP-run statehouse in Bismarck against tribal leaders and voting rights groups. Census Bureau records show 46,000 Native Americans live in North Dakota, including 20,000 on tribal reserves. According to court filings, at least 5,000 of those on reservations do not have conventional addresses.

North Dakota has become a high priority for Republicans as they seek to retain their slim majority in the Senate. In television interviews this week, Trump, who won the state by 36 percentage points in 2016, touted polls showing Rep. Kevin Cramer (R) leading Heitkamp by double digits. Republicans have made Heitkamp’s vote against the confirmation of Supreme Court Justice Brett M. Kavanaugh a centerpiece of their effort as the campaign enters its final straight.

Native American voting right groups described this week’s Supreme Court ruling, in which Kavanaugh did not participate, as the latest ploy to depress already-low turnout among tribes.

“It is a partisan and intentional effort at targeting native voters,” said Matt Campbell, attorney at the Native American Rights Fund, who represented a group of military veterans as plaintiffs in the case. He cited official figures indicating that in 2012 there were just nine cases of potential voter fraud out of 325,862 votes cast. “Voter fraud is not a problem,” he said.

Al Jaeger, North Dakota’s secretary of state, did not respond to a request for comment. However, in a reply to Four Directions obtained by The Washington Post, Jaeger warned that the activists’ plan could cause confusion among voters. He cited the example of the Turtle Mountain Band of the Chippewa Tribe, which he said was already bound by an agreement delegating authority for erecting road signs and assigning street addresses to Rolette County.

In a 2017 letter to President Trump’s now-defunct Commission on Election Integrity, Jaeger also laid out his views on the need for stricter voter ID requirements.

“While some individuals argue that there is no evidence of widespread voter fraud, there are others who argue the exact opposite,” Jaeger wrote. “Regardless, the truth is that under the current forms of election administration, it is not possible to establish whether widespread voter fraud does or does not exist because it is difficult to determine either way when proof is not required of voters when registering or before voting.”

In a dissenting opinion in the Supreme Court case, Justice Ruth Bader Ginsburg warned that the North Dakota ID law presented a “severe” risk of “grand-scale voter confusion.” Her opinion noted different ID rules were in place during the state’s primary election this spring, meaning some voters might show up at the polls with ID that falls short or simply stay at home in November.

“Reasonable voters may well assume that the IDs allowing them to vote in the primary election would remain valid in the general election,” Ginsburg wrote.

Cramer has not publicly responded to the Supreme Court ruling, and his campaign did not reply to a request for comment from The Washington Post.

Julia Krieger, communications director for the Heitkamp campaign, declined to comment on the plans by Native American activists but described the voter ID law as a partisan move aimed at lowering turnout among Heitkamp’s voters.

“Passed by North Dakota’s Republican-majority legislature almost immediately following Heidi’s victory in 2012, it’s no secret that North Dakota’s hyperpartisan voter ID laws target student and Native communities because they prefer Heidi in the U.S. Senate,” Krieger said.

While North Dakota has voted Republican in 19 out of the past 20 presidential races, the state was long represented by two Democratic senators, Byron Dorgan and Kent Conrad. Heitkamp ran in 2012 after her mentor, Conrad, encouraged her; she has remained popular in part by distancing herself from national Democrats, although the state has been trending to the right in recent years.

The first flash point in the legal battle over voter ID laws came in 2013 in the wake of Heitkamp’s election, when the state legislature argued the system in place at the time facilitated voter fraud. Legislators banned alternatives for those without ID, including affidavits signed under penalty of perjury or tribal officials testifying that a voter was a local resident. They then removed college and military cards from the list of acceptable documents and passed another law requiring that a person’s ID contain a current residential address.

Jim Silrum, North Dakota’s deputy secretary of state, a proponent of the efforts to tighten voter rules, said at the time the measure was drafted after “concerned citizens” and state representatives raised fears about the possibility of voter fraud.

Activists say a succession of laws put in place since 2012 have disenfranchised thousands of tribal voters, especially those who lived on reservations or in wilderness areas. During the 2014 midterms in Rolette County, home to the Turtle Mountain tribal reservation, turnout plunged from 45 percent to 33 percent, while neighboring non-tribal areas saw no comparable decline.