The Democrats have handed in a request for a special parliament session
The main opposition party in Thailand has called for an emergency session of parliament to prove it has sufficient support to form a new government.
The Democrat Party will propose that its leader, Abhisit Vejjajiva, becomes the next prime minister.
Thailand's ruling party was dissolved last week by the courts, after being found guilty of electoral fraud.
But some of its members have formed a new party, Phuea Thai, and insist they can also form a new government.
The BBC correspondent in Bangkok says any administration that emerges is likely to be a messy coalition.
The power struggle at the heart of Thai politics comes after a tumultuous few months in which anti-government protesters campaigned to bring down the prime minister and the whole administration.
They claimed the ruling People Power Party (PPP) was merely a proxy for ousted prime minister Thaksin Shinawatra.
They recently blockaded Bangkok's main international airport for more than a week, leaving 300,000 foreign travellers stranded and dealing a huge blow to the tourist industry.
The Democrats want their leader to be the new prime minister
The blockade only ended when the Constitutional Court ruled that the PPP had committed electoral fraud, and barred Prime Minister Somchai Wongsawat and other senior party members from politics.
Now the battle has moved to parliament, and the Democrats, who lost to the PPP in the last general elections in December, are seizing their chance.
They say they have wooed several smaller parties away from the PPP, and have enough support to form a coalition government.
Democrat Party whip Sathit Wongnongtoey told the French news agency AFP: "The party will ask for parliament to reconvene in an extraordinary session... the name who will be proposed as prime minister is Abhisit [Vejjajiva]."
Representatives from some small parties have confirmed they have switched sides, but analysts say that allegiances could easily change back again - something that Puea Thai is hoping will happen in the coming days.
Puea Thai has not yet named its candidate for prime minister.
Adding to the sense of crisis around Thailand's ongoing political struggles, Thailand's deeply revered King Bhumibol Adulyadej was too sick to make his traditional speech on the eve of his birthday on Thursday.
Thais had been looking forward to hearing from their much-loved monarch, who has long been looked to for guidance in times of turmoil.
Another recent development is the return of Thaksin's ex-wife to Thailand on Friday.
She claims to have gone back for personal rather than political reasons.
But news of her return surprised analysts, because she was given a three year jail term in July, after being found guilty of tax evasion, and left the country while on bail after filing an appeal.