Ex-PM elected opposition party head
Mr Abhisit ran unopposed in the party election, and received 98% of the vote.
He was charged with murder last week in connection with the deaths of two people during a 2010 crackdown on anti-government protesters when he was prime minister. A total of 91 people died during the protests.
The Democrats must decide soon whether to participate in the February 2 polls called by prime minister Yingluck Shinawatra.
They have not won an election in decades, and have allied themselves with a protest movement that is demanding that Ms Yingluck resign before any polls to make way for an appointed government to institute reforms.