Bookmakers give Sweden's entry, comedy trio KAJ, a 42 per cent chance to win the world's biggest music event, which is taking place amid protests over participation by Israel, which is continuing a military assault on the Palestinian enclave of Gaza.
A win for KAJ would be Sweden's eighth overall, moving it clear of Ireland to become the most successful country in Eurovision history.
KAJ's song, Bara Bada Bastu, translates as "Let's Just Sauna" and celebrates the power of saunas.
"It is a cherished way of everyday life, just relaxing and connecting and like staying quiet," KAJ member Kevin Holmstrom told Reuters before the final.
Austria, represented by singer JJ who combines elements of opera and techno in his song Wasted Love, is second favourite.
"If you look at the bookies, it's a two-horse race," said Eurovision expert William Lee Adams, who called KAJ's song "fun, silly and camp" - key ingredients for Eurovision success.
"You've got people dancing in towels and you've got men in suits at the start, very uptight but by the end of their act, they're loose and ready to go. It's feel-good fun."
More than 160 million people are expected to watch Eurovision, which could feature an appearance by Canadian megastar Celine Dion, who won in 1988 representing Switzerland.
Dion, who may be unable to attend due to health problems, said she would love nothing more than to be in Basel in a video message at Tuesday's semi final.
Switzerland won the right to host Eurovision after rapper and singer Nemo won last year's contest in Malmo, Sweden.
The event, which stresses its political neutrality, has again faced controversy this year due to the war in Gaza.
Israel's entrant, Yuval Raphael, was at the Nova music festival during the October 7, 2023, attack by Hamas militants on southern Israel that killed 1,200 people and saw 251 taken hostage, according to Israeli officials.
Pro-Palestinian groups urged the European Broadcasting Union to exclude Israel over Gaza, where more than 50,000 people have been killed in the ensuing offensive by Israel, according to local health officials.
They accuse EBU of hypocrisy by allowing Israel to compete while Russia was excluded following its invasion of Ukraine.
"How can Israel compete in a happy competition like Eurovision, which celebrates tolerance, when it is killing thousands of people and bombing hospitals in Gaza?" said Geri Mueller, president of the Association Swiss Palestine.
The EBU said it hoped the contest could overcome divisions.
