BANGKOK — Thailand’s main opposition party said on Saturday that it would boycott national elections scheduled for February, strengthening its alliance with the tens of thousands of antigovernment protesters who have rallied on the streets of Bangkok for the past month.

Abhisit Vejjajiva, the leader of the Democrat Party, which is Thailand’s oldest political party and has its power base in the country’s old moneyed elite, said that politics was at a “failed stage” and that the elections would be the “same old power grab” by the governing party and its allies.

“The election on Feb. 2 is not the solution for the country,” Mr. Abhisit, a former prime minister, said after meeting with party leaders on Saturday. “It will not lead to reform.”

The Democrat Party and the protesters are deeply frustrated by the electoral power and influence of Thaksin Shinawatra, a tycoon who founded the country’s most successful political movement and whose sister, Yingluck Shinawatra, is prime minister. They accuse Mr. Thaksin of subverting democracy through corruption and populist policies.