Starmer's Labour Party is bracing for big losses in voting that will choose about 5000 local councillors and a handful of mayors across England, as well as semi-autonomous parliaments in Scotland and Wales.
Polled opened on Thursday morning, with the bulk of the results likely to be declared on Friday afternoon.
Local elections usually focus on issues such as rubbish collection, graffiti and potholes, but Starmer's opponents have painted the vote as a referendum on the prime minister.
A rout could trigger moves by restive Labour MPs to oust a leader who led them to power less than two years ago.
Even if Starmer survives for now, many analysts doubt he will lead the party into the next national election, which must be held by 2029.
Starmer's popularity has plunged after repeated missteps since he became prime minister in July 2024.
His government has struggled to deliver promised economic growth, repair tattered public services and ease the cost of living - tasks made harder by the US-Israeli war with Iran, which has choked off oil shipments through the Strait of Hormuz.
The prime minister has been further hurt by his disastrous decision to appoint Peter Mandelson, a scandal-tarnished friend of Jeffrey Epstein, as Britain's ambassador to Washington.
Labour is defending about 2500 seats on English councils, and party members are apprehensive it might lose many of them.
An election rout could trigger a snap leadership challenge or internal party pressure on Starmer to step down.
He has already survived a crisis in February, when some Labour MPs, including the party's leader in Scotland, urged him to quit over the Mandelson appointment.
The big winner is expected to be hard-right party Reform UK, led by Nigel Farage, which is aiming for working-class, former Labour strongholds in England's north and on London's outer edges with its anti-establishment, anti-immigration message.
The Green Party is also likely to gain hundreds of council seats in urban centres and university towns.
The main opposition Conservative Party is also expected to lose ground, with the centrist Liberal Democrats making some gains.