One vote can change everything, especially when things are close.

Voters outside Charlotte, North Carolina, had every reason to expect their representative would vote to protect abortion rights.

State Rep. Tricia Cotham once shared the story of her own abortion in a speech supporting abortion rights on the state House floor.

Earlier this year, she signed on to a bill that would codify abortion rights under Roe v. Wade in her state. Cotham is still listed as a co-sponsor.

But then, earlier this month, she voted for a ban on most abortions after 12 weeks of pregnancy, about half of what was guaranteed under Roe v. Wade.

From Democrat to Republican

What happened in between is that Cotham switched parties and upended North Carolina's power balance in the process.

Alienated by Democrats, in particular on the issue of education and her deeply held support for charter schools, Cotham said she no longer felt represented by the party. She became a Republican in April and gave Republicans a veto-proof majority in the state House. Cotham did not respond to a request for comment.

Republicans are now poised to use that majority to change abortion law in the state over Democratic Gov. Roy Cooper's objection. Cooper vetoed the bill on Saturday, but a vote to override that veto is now scheduled for Tuesday.

"If just one Republican in either the House or the Senate keeps a campaign promise to protect women's reproductive health, we can stop this ban," Cooper told supporters at a campaign-style rally in Raleigh on Saturday.

Republicans have argued the bill represents compromise -- most US abortions occur in the first trimester. But the bill also places new restrictions on women seeking an abortion before that 12-week threshold. Read CNN's full report.

Lawmaker who missed vote had supported current law

Another Republican lawmaker, state Rep. Ted Davis, was absent when the abortion bill passed through the North Carolina House.

He has also previously expressed support for leaving North Carolina law where it is, with women able to make their own choice for up to 20 weeks of pregnancy, according to previous comments circulated by the governor's office.

RELATED: See where abortion is banned, where it's legal and where it's in limbo

Variations on 'pro-life'

Nearly all Republican lawmakers these days describe themselves as "pro-life," but the term can mean drastically different things.

Abortion remains legal in South Carolina for up to 20 weeks of pregnancy, for instance, because five women voted against a near-total abortion ban last month -- including three Republican state senators who bucked the rest of the party. The ban was defeated by a single vote.

South Carolina state Sen. Penry Gustafson, who helped defeat that ban, told CNN's Christiane Amanpour this month that she considers herself to be "pro-life," but that neither she nor most South Carolinians support a total ban.

"The Republican consensus is that we do need restrictions," Gustafson said. "There's a range of that between zero weeks and 12 weeks." She supports a six-week ban in South Carolina and opposes a federal abortion ban. Watch Amanpour's interview of Gustafson.

Trying again for new restrictions in South Carolina

South Carolina Gov. Henry McMaster, a Republican, has called the state legislature back for a special session Tuesday to take another crack at new abortion legislation. A six-week abortion ban passed by the state in 2021 has been struck down by the state's Supreme Court. Read more about developments in South Carolina.

The current ban in South Carolina, which makes most abortions illegal after about 20 weeks, exists because another Republican woman, former Gov. Nikki Haley, signed it into law back in 2016. When Roe fell, the law Haley signed went into effect.

A call to find consensus

Now running for president, Haley supports more restrictions on abortion than the 20-week ban she had signed into law. It was, she said on CBS News on Sunday, "the furthest we could get it at the time."

But her focus now is on national consensus rather than imposing a nationwide ban like other likely Republican presidential candidates have suggested.

"Why try and divide people further?" Haley said, noting that neither party will get the 60 votes needed to impose a ban or codify Roe v. Wade. "Why not talk about the fact that we should be trying to save as many babies as possible and support as many mothers as possible? I think the media has tried to divide them by saying we have to decide certain weeks. In states, yes. At the federal level, it's not realistic. It's not being honest with the American people."

She's right about neither party likely getting 60 votes. And that means the abortion rights map will continue to evolve, and not just in the South.

Getting ahead of voters in Ohio

CNN's Fredreka Schouten reports that the GOP-controlled legislature in Ohio, dominated by opponents of abortion rights, has scheduled an August election to make it more difficult for voters to change the state's constitution.

That move by the legislature, conveniently, gets ahead of abortion rights supporters, who are gathering signatures for a ballot measure in November over whether to guarantee abortion rights in the state.