In the 1980s the Rajneeshee cult attempted this on a much smaller scale in Wasco County, Oregon. That's the classic cultists in orange robes while their aloof leadership drives around in Rolls-Royces. They weren't partisan in your sense, but they illustrate how it can be done, and how it can be stopped.
Rajneesh and thousands of his followers moved to Wasco County with a population of just 22,000. They incorporated themselves into the city of Rajneeshpuram, zip code 97741. Further expansion of Rajneeshpuram was mired in legal conflict with claims that it was essentially property of a religious organization and its incorporation violated separation of church and state, as well as investigations into immigration violations.
They looked to the nearby tiny town of Antelope, Oregon (population then about 50). The cultists purchased lots in Antelope, registered to vote, and took over the town government by overwhelming the small number of local residents. Now they had a more legitimate town government to work with and renamed the town Rajneesh.
Facing further local and state backlash, they turned to influencing the November 1984 Wasco County election. Their plan was to control the county by getting their members elected to a majority of the County commissioners as well as the sheriff. They attempted to get candidates on the ballot, but failed to get sufficient valid signatures.
Wasco County was too large to take over directly as with Antelope. Many of the commune's members were not US citizens. Oregon only required 20-days of residency before being allowed to vote. The commune attempted to boost their voter registration with a "Share-a-Home" program; they brought in thousands of houseless people to live in Rajneeshpuram and tried to get them to register them to vote. Many refused and left, citing an atmosphere of brain-washing and feeling they had been tricked. A month before the election County Clerk Sue Proffitt, citing evidence of fraud, halted new registrations. Oregon Secretary of State Norma Paulus required new voter registrations to have a hearing in the county seat. At the cut-off date for registration, the Rajneeshees halted their imports of houseless people.
At the first hearing just 14 voters of about 200 were registered. At the second hearing they didn't show up. They Rajneeshees decided to boycott the election. The effort had failed.
Instead, the Rajneeshpuram tested their backup plan: make enough voters sick so they won't show up to vote. They cultured Salmonella and made "test runs" poisoning two county commissioners, spreading it throughout grocery stores, court houses, and salad bars sickening and hospitalizing hundreds of people. Fortunately there were no fatalities. This only boosted voter participation to keep the cultists out.
The Rajneesh leadership fled to Europe. They were extradited from West Germany and sentence to 20-years, but were released after 3 and moved to Switzerland. The mayor of Rajneeshpuram turned state's evidence. The Rajneesh himself outed their crimes claiming he was unaware at the time; he was arrested on immigration crimes and deported. He died in 1990 in India.
This was the first and largest bioterror attack in the US.
Going back to the question, it illustrates some of the powers county clerks and secretaries of state have to combat overt voter fraud via registration.