It took 150 years for Florida’s ex-felons to win back their right to vote – and a single swish of the Republican governor’s pen last month to seemingly snatch it away again.

Now supporters of Amendment 4, which ended the civil war-era disenfranchisement of released criminals by attracting a two-thirds majority in a public ballot initiative last fall, have regrouped for a multifaceted fight to reimpose the will of the people.

The most significant step is a federal lawsuit to challenge what advocates say is an illegal “poll tax” placed on up to 1.5 million so-called “returning citizens” by the Florida legislature this summer. The fight over who can vote is one that could shape the political future of the state and have a huge impact on the 2020 election.

Amendment 4 allows ex-felons not convicted of murder or sexual offenses to vote once their sentences were completed. But Republican politicians crafted a bill creating obstacles: Senate Bill 7066, signed into law by Ron DeSantis, the governor, in June, that now requires the former felons to have met all outstanding legal financial obligations, including court fees, often running to thousands of dollars, before being able to register.

As a consequence, hundreds of thousands of ex-felons who were able to rejoin the voting rolls when Amendment 4 passed in November now face being struck off again in a major setback for voting rights advocates.

“Over a million Floridians were supposed to reclaim their place in the democratic process, but some politicians clearly feel threatened by greater voter participation. They cannot legally attach a price tag to someone’s right to vote,” said Julie Ebenstein, senior staff attorney with the American Civil Liberties Union (ACLU), in announcing the lawsuit on behalf of 10 affected ex-felons.

Advocates are also finding other ways to fight back.

The Florida Rights Restoration Coalition (FRRC), which led the campaign for November’s historic vote, this week launched a fighting fund to help felons pay off their fines and fees, raising more than $120,000 of its initial $3m target in the first few days.

The group is also working with judges and court officials to find an easier pathway to the ballot box. Andrew Warren, the state attorney for Hillsborough county, which includes the city of Tampa, has proposed a system in which judges could convert the fines and fees of lower level, non-violent offenders into community service, allowing them to discharge their financial obligations and again become eligible to vote.

“Our understanding is there’s 500,000-plus people who are not yet eligible, they’re no longer dealing with a lifetime ban [from voting] because of their felony convictions, but they are primarily obstructed by financial obligations,” said Neil Volz, deputy director of the FRRC, which has hosted a series of public meetings around the state to discuss the implications of the new Florida law.

“One person owes $800 here, this person has $300 there, that person $400 and so on. We are trying to connect with each other, you’re beginning to see community connecting with community. We want to help people move on with their lives.”

One of the plaintiffs in the lawsuit, Keith Ivey, 46, was released from prison seven years ago, having served eight and a half years for identity theft and property crimes. He runs a Jacksonville car dealership with his father, employing 20 people, and was among the first to register when Amendment 4 passed. He voted in two local elections this spring, but discovered only afterwards that he still owes $400 in court fees from more than 20 years ago, disqualifying him again.

Before the Amendment 4 ballot, he appeared in campaign videos for a yes vote.

“Without second chances, without society allowing returning citizens an opportunity to regain a foothold in their lives, there’s no hope,” he said. Now, he told the Guardian, he has been worn down by the process.

“I’m just exhausted by it all,” he said. “I went and registered, got my card, I actually voted in two local elections, and then two weeks ago the governor signed the bill to change the language.

“The system is not doing what it was prescribed to do, it’s a whole bunch of political jockeying and the little man on the totem pole does not have a voice. I’m still in the fight but I’m truly exhausted from the unfairness of it all. Politics have gotten involved and now it’s about upper hand and leverage.”

The lawsuit, filed jointly by the ACLU, League of Women Voters and National Association for the Advancement of Colored People, says the Florida law creates a “poll tax” contrary to supreme court rulings prohibiting the restriction of a person’s right to vote in federal or state elections on financial grounds. It also claims the law is partly racially motivated.

“It creates two classes of returning citizens, those who are wealthy enough to vote and those who cannot afford to,” the lawsuit states. “This disenfranchisement will be borne disproportionately by low-income individuals and racial minorities, due to longstanding and well-documented racial gaps in poverty and employment.”

A 2016 study by The Sentencing Project found that more than one in five African Americans were disenfranchised in Florida.

The Florida governor’s office did not return a request for comment, but DeSantis said in May that he considered the “idea that paying restitution is equivalent to a tax is totally wrong”.

“The only reason you’re paying restitution is because you were convicted of a felony,” he said.