Voters in Alabama headed to the polls in a hard-fought U.S. Senate race with high stakes for President Donald Trump.
Mr Trump has endorsed fellow Republican Roy Moore in spite of allegations against him of sexual misconduct toward teenagers.
Mr Moore, 70, a former Alabama Supreme Court chief justice, is battling former US attorney Doug Jones, a Democrat, for theSenate seat.
Mr Jones, 63, hopes to pull off an upset victory in the deeply conservative Southern state.
The race will test Mr Trump's political clout after nearly a year in office, with his approval ratings at historically low levels.
A win by Mr Moore would strengthen Mr Trump's grip on the Republican Party, some of whose leaders have not backed Mr Moore.
A Jones victory could mean trouble for Mr Trump and his populist political base.
It would certainly narrow the Republicans' already slim majority in the US Senate, possibly making it harder for Mr Trump to advance his policy agenda.
The sexual misconduct accusations against Mr Moore come amid a wave of such allegations against powerful men.
Democrats have signaled that, if Mr Moore wins, they will try to tar Republicans as insensitive to women’s concerns.
Polls opened at 7am (1pm Irish time) in the special election to fill a seat vacated by Republican Jeff Sessions, who became US attorney general in the Trump administration.
Mr Moore, a lawyer who often wears Western-style clothing, showed up to vote at the Gallant Fire Department in northern Alabama on horseback and wearing a cowboy hat.
.@MooreSenate arrived at his polling place on horseback today as voters in Alabama decide the special Senate race. We're discussing the election on @drivetimeRTE shortly. @rtenews pic.twitter.com/cZRr4m8Rkk— Caitriona Perry (@CaitrionaPerry) December 12, 2017