Voters in Chumphon province may be unable to go to the polls on Sunday after election officials failed to obtain ballots.
Provincial Election Commission (EC) officials were unsuccessful in negotiating with protesters to release the ballots.
Around 2,000 protesters from the People's Democratic Reform Committee (PDRC) have surrounded the entrance of a border patrol police camp where the ballots have been stored.
The protesters want reform before any election can take place, in line with the demands of PDRC secretary-general Suthep Thaugsuban.
Prachin Tharnsirisi, chairman of the EC for Constituency 1, went with police to negotiate with the PRDC and the Chumphon post office to seek their cooperation to allow the distribution of ballots to election units.
The protesters stood firm on their position so the EC officials left without obtaining the ballots.
Chumphon is one of eight southern provinces with a total of 28 constituencies where candidates were unable to register because of blockades by protesters.
If voting in those provinces can go ahead, voters will be able to cast party-list ballots only. The affected provinces are Nakhon Si Thammarat, Chumphon, Surat Thani, Krabi, Trang, Songkhla, Phuket and Phatthalung.
Voting in some other southern provinces is also in jeopardy because of a lack of ballots, while the EC also faces a severe shortage of poll officials in many parts of the country.
Protesters are also blockading post offices in Nakhon Si Thammarat and Songkhla where party-list ballot papers for 14 southern provinces are being held.
If the ballot papers cannot be delivered, no voting would take place in the affected areas but another attempt would be made on March 2, the EC has said.
Chumphon Election Commission officials confer outside a blockaded police camp where ballots were being stored on Saturday.