Abortion advocates in Kentucky are poised for an underdog victory, as the final votes are counted in a state referendum on an anti-abortion measure.

The result here will follow three other pro-choice victories in this year's midterm elections: Vermont, Michigan and California have all enshrined the right to abortion in their constitution.

However, in Kentucky the expected result won't automatically reverse the state's current legislation, which almost entirely prohibits abortion.

The wave of ballot initiatives comes in the wake of the June Supreme Court decision to overturn Roe v Wade, ending a national guarantee to abortion access.

In a hotel ballroom in downtown Louisville, pro-choice advocates milled around, the roomed flanked by bright yellow balloons, emblazoned with the words "Bans off our bodies".

It had become clear they were poised to win, defeating a ballot measure that would have explicitly removed abortion rights from Kentucky's constitution.

"Abortion is a winning issue," said Elizabeth Nash, a policy analyst with the Guttmacher Institute, a pro-choice research group.

"Voters want people to have bodily autonomy and agency over their lives. That is pretty core to being an American."