Key Pheu Thai Party figure Sudarat Keyuraphan is reportedly supporting Pol Maj Gen Supisarn Phakdinaruenart, former commander of the Crime Suppression Division (CSD), in his contest at Sunday's senate election.
Pheu Thai spokesman Prompong Nopparit, however, denied that Pheu Thai is supporting Pol Maj Gen Supisarn.
Party sources said Ms Sudarat met former Pheu Thai MPs last week to discuss the election.
At the meeting, MPs were told to support Pol Maj Gen Supisarn, the sources said.
The former CSD chief belongs to Class 31 at the Royal Police Cadet Academy like Pol Gen Pongsapat Pongcharoen, a former Bangkok governor candidate and Pheu Thai member who is currently secretary-general of the Narcotics Control Board, and assistant national police chief Somyos Phumphanmuang.
Jirayu Huangsap, a former Pheu Thai MP for Bangkok, admitted that a meeting took place between former Pheu Thai MPs, but he said the MPs were not told how to vote in the upcoming election.
Mr Jirayu said he personally supported Pol Maj Gen Supisarn because he was experienced in crime suppression and his knowledge would be useful in serving the public.
Mr Prompong said Pheu Thai was not backing Pol Maj Gen Supisarn and said the party instructed its members to remain neutral as public support for a senatorial candidate is illegal.
Advance voting for senators happened nationwide yesterday ahead of Sunday's vote.
It went ahead without disruptions like those seen at the Feb 2 election, however the turnout was small.
Puchong Nutrapong, secretary-general of the Election Commission, said 2,121,814 people had registered for advance voting outside their constituency and 36,185 people had registered for advance voting in their constituency.
However, only about 88,000 people, or 4.18% of registered voters cast their ballots yesterday.
The figure was far below advance polling in the previous senate election in 2008, when more than 26% of advanced voters cast ballots.
Mr Puchong said polling stations had opened without any problems, except some polling stations in Phayao province that were damaged by storms. They eventually reopened when the storms died down.
Five election commissioners will meet today to discuss advance voting, especially the very low turnout.
They will ask the media to help raise public awareness for the senate election on Sunday.
In Chumphon, about 5,000 people registered for advance voting but only 126 turned out to vote. In Prachuap Khiri Khan, about 14,670 people had registered for advance voting but only dozens showed up at polling stations.
In Samut Prakan yesterday, polling stations were quiet partly because local election authorities changed the voting venue from its usual location at the Royal Thai Navy Academy to the Benja Burapha Suvarnabhumi Centre, where a small number of passenger buses pass.
Most people who had registered for advance voting in Samut Prakan are natives of other provinces and half of them came from the Northeast.
In Phitsanulok, handfuls of voters showed up at polling stations at the district office and Pibulsongkram Rajabhat University.
Of 655,558 eligible voters, about 9,000 had registered for advance voting and most were natives of other provinces.
In Trang, about 3,000 people registered for advance voting but only about 300 voted yesterday.
Polling stations in Kanchanaburi were also quiet.
Many eligible voters who intended to cast their vote yesterday showed up only to find that their names had been excluded from the voting register. Most are soldiers of the 9th Infantry Division.