•Ohio's measure, known as Issue 1, was widely seen as the toughest fight so far for abortion rights supporters as it was the first Republican-led state to consider changing its constitution to explicitly guarantee the right.
Ohio voted to enshrine abortion rights in its constitution on Tuesday, marking a major victory for pro-choice campaigners in the state.
The BBC's US partner CBS News projected a decisive victory for the amendment.
Early returns showed almost 58% of voters in the conservative-leaning state had backed it.
Its projected success is likely to bolster Democrats' hopes that abortion rights remain a winning issue ahead of elections in 2024.
It also extends an unbeaten record for ballot measures designed to protect abortion rights since the nationwide right to the procedure was rescinded by the Supreme Court last year. This is the seventh such measure to pass.
But Ohio's measure, known as Issue 1, was widely seen as the toughest fight so far for abortion rights supporters as it was the first Republican-led state to consider changing its constitution to explicitly guarantee the right.
Supporters of the amendment warned voters that unless it passed, more restrictive laws could be introduced including a ban on abortions after six weeks of pregnancy with no exceptions.
Abortion is currently legal in Ohio until 22 weeks of pregnancy.
Those who campaigned against the measure, however, argued that it would allow late term abortions that are currently illegal.
As votes were still being counted on Tuesday night, there were emotional celebrations from pro-choice supporters as US media projected the constitutional amendment would pass.
"This is one of the greatest moments of my life, working so hard with my team beside me to achieve reproductive rights and freedoms in Ohio," Kate Gillie told the BBC at one watch party.
"We've got two little girls and this is about their future and their reproductive rights," another person at the party, Frank Tedeschi, said.
One of the leading groups that opposed the amendment, Protect Women Ohio, has reportedly raised almost $10m (£8.1m) since September.
In a statement, the group said: "Our hearts are broken tonight not because we lost an election, but because Ohio families, women and children will bear the brunt of this vote."
"We stand ready during this unthinkable time to advocate for women and the unborn," it added.
The amendment will change the state's constitution to include protections for abortion access. It will establish "an individual right to one's own reproductive medical treatment," including on abortion, contraception and miscarriage care.
It explicitly prohibits the state from "directly or indirectly burdening, penalising or prohibiting abortion" before viability, which is generally considered to be around 23 weeks of pregnancy.
The amendment does allows the state to bar abortion after the point of viability except in instances where the patient's doctor determines the procedure is needed to protect life or health.
Opponents of the measure have expressed concern over this element, telling voters it would allow for "late term abortions" - a non-medical term referring to abortions later in pregnancy.
But supporters of Issue 1 argued that any abortions later in pregnancy would require sign-off from a medical professional attesting to serious health concerns.
The result in Ohio, meanwhile, may offer clues about voters' views on abortion more than a year after Roe was overturned.
"If the ballot initiative passes really easily it will confirm that voters are still mad," said Mary Ziegler, a law professor at the University of California, Davis, and a leading expert on the US abortion debate, before polls closed. "But it won't confirm abortion is a priority issue for them, that's a different question."
Democrats, including President Joe Biden, will hope the issue continues to energise voters ahead of next year's elections.
"Ohioans and voters across the country rejected attempts by MAGA Republican elected officials to impose extreme abortion bans," Mr Biden said in a statement on Tuesday night.
Two other elections on Tuesday, one in Kentucky, the other in Virginia, will also impact abortion access in the months ahead.
In the southern state of Kentucky, the re-election of Democratic Governor Andy Beshear will be considered a win for activists fighting to maintain abortion access in the state.
And in Virginia, Republicans are seeking total control of the legislature. If they are successful, the party is expected to pursue new restrictions on the procedure.