Brnabic, nominated last week by Serbian Present Aleksandar Vucic, could also become the conservative nation's first female prime minister if she's selected. But some of Vucic's coalition partners said they will vote against her because of her sexual orientation.
Vucic on Monday met with 100 of his Serbian Progressive Party, or SNS, lawmakers who promised to vote for Brnabic in Serbia's 250-seat parliament. She needs at least 26 more votes to be confirmed.
After the closed meeting, Vucic only said, "everything will fine."
The vote was originally scheduled for later this week, but it has been now moved to next week because of the uncertainty.
Serbian Progressive Party official Marija Obradovic said that if Brnabic doesn't get the additional support by Thursday, an early vote will be called. That would be the third snap election in Serbia in five years.
The head of Vucic's party in the assembly, Aleksandar Martinovic, said "those who abstain from voting, or those who take a stance against Brnabic, will in fact take a stance against Vucic."
He said that the coalition parties should "consider carefully" about how they vote, adding that "those who do not vote (for Brnabic) will exclude themselves from the coalition with SNS."
U.S.-educated Brnabic, 41, was a minister in Vucic's government before he became president in April. Although not a member of his party, she is considered to be loyal to the autocratic leader.
Brnabic's nomination was considered part of Vucic's tactics to please the West amid his recent apparent shift toward Russia despite officially claiming European Union membership is Serbia's "strategic goal."
Serbia's LGBT population has often faced discrimination, harassment and intimidation.
If approved, Brnabic will join a handful of other openly gay leaders in Europe. Earlier this month, Ireland selected Leo Varadkar to head their governing party who will lead the government.
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