Democrat deputy leader Sathit Pitutecha denies planning to leave the party despite saying its popularity is in decline and its leader lacks the charm to impress voters.
He was responding to reports saying he had been approached by the Bhumjaithai Party to join its ranks.
In an interview with the Inside Thailand news programme, he said there has always been speculation about him switching to another party but he has long ruled out such a move.
The latest rumour was apparently triggered by last week's meeting between Bhumjaithai Party leader Anutin Charnvirakul and former Democrat Party leader Abhisit Vejjajiva, which Mr Sathit also attended.
The reports came as the Democrat Party was thought to be bleeding members ahead of the next general election, tentatively set for May 7.
Mr Sathit said he was approached by the Bhumjaithai leader about joining its ranks but declined the offer.
"He asked me politely because he knows I've been in this party for years," he said, referring to Mr Anutin.
"I thanked him but told him that I couldn't be in his party."
"Bhumjaithai has welcomed a lot of people. I'm still here [in the Democrat Party], of which I'm also a deputy leader," he said.
"I thanked [Mr Anutin] for offering me the honour and I still respect him."
Mr Sathit, who is also deputy public health minister, admitted the Democrat Party's popularity is in decline.
However, he said regardless of the reason for the slump, Democrat principles remain firmly intact.
"Our party's rating isn't good. It could be because of the political situation, or the party leader's uncharismatic, even though he works really hard," he said.
"He is quiet, which possibly makes him not so popular.
"But when compared with other parties, the Democrat Party has upheld its values -- that the party isn't owned by anyone and can't be told what to do," he said.
While noting that defections are quite common ahead of an election, Mr Sathit urged the public to see for themselves if such moves are made in the interest of politics or are self-serving.
In a phone interview with the programme, Mr Anutin downplayed Mr Sathit's remarks.
"He has a promising future in the [Democrat] Party. Why should we take him away?" Mr Anutin said.
"Bhumjaithai has a lot of able people. I don't think it has room to accommodate him."
He said last week's meeting with Mr Abhisit was about work and politics, without going into detail.
Mr Sathit said in a media interview on Tuesday that the meeting had no political implications.
