As the Thai Rak Thai Party continues its election campaigning, a deputy leader has admitted it will be difficult for the party to meet the minimum vote requirement in some constituencies, a hurdle that could trigger a fresh round of voting in those seats.

In a constituency where only one candidate is running, the candidate must win at least 20 per cent of the eligible votes to secure the seat, Article 74 of the election law stipulates. If the candidate falls short, new polls must be held until he or she meets the 20 per cent  minimum. A new government cannot be formed until 500 candidates have been elected.

TRT deputy leader Chamlong Krutkhunthod said after registration closes for constituency candidates on Wednesday the party will identify constituencies where its candidates face no opponents and then step up efforts to persuade voters in those areas to cast ballots for TRT candidates.

Chamlong identified the South, the Democrat Party's stronghold, as an area of concern.

Sa Kaew, the political base of rebel TRT faction Wang Nam Yen, and Suphan Buri, the Chat Thai Party's base, were also singled out.

The Democrat, Chat Thai and Mahachon parties are boycotting the election.

Seven parties registered 252 candidates on the first registration day for constituency-based candidates yesterday. Candidates can register to run in the 400 constituencies or seek a place on a party list for which 100 seats in the 500-seat Parliament are reserved. After the election, each party is given seats for party-list candidates according to the proportion of votes it receives nationally.

No constituency candidates registered in Sa Kaew province yesterday.

The son of Wang Nam Yen leader Snoh Thienthong, former Sa Kaew MP Sorawong, said it was doubtful a TRT candidate would be able to win 20 per cent of the votes in a constituency in his family's political base.

"If there is only one TRT candidate I think the candidate may not receive 20 per cent of all eligible votes," Sorawong said.

Chat Thai deputy leader Jongchai Thiengtham said it would not be easy for TRT to win 20 per cent of eligible votes in every constituency in Suphan Buri.

"In the past, candidates from other parties who ran in Suphan Buri received only 5-10 per cent of the votes," he said.

Some small parties have been accused of working for TRT by fielding candidates so that the TRT would not face the 20 per cent snag.

The former ruling party's major concern is the possibility that there will be many constituencies where it will have to run solo, especially in the South.

Some regional parties, including the Prachakorn Thai Party, have said they will expand nationwide for this election.

The Paen Din Thai Party yesterday announced it would field candidates all constituencies of the 14 southern provinces.

The Election Commission said the seven parties had fielded candidates in 56 provinces, but no candidates had been registered for 20 provinces.

The 20 include Krabi, Kamphaeng Phet, Chai Nat, Chiang Rai, Lampang, Sa Kaew and Satun.