Just four months after forming a government, Thailand's elected Premier Samak Sundaravej is under attack both in the streets and in parliament, raising new fears about the nation's stability.

He has faced nearly four weeks of small but persistent street protests, which received a boost Tuesday when Thailand's most powerful unions gave their backing to the rallies.

The opposition Democrat Party on Wednesday lodged a motion of no confidence against him, while rising fuel and food costs have raised threats of protests by farmers and truckers.

The tumult has cast a shadow over Samak's victory in December elections, which ended more than a year of military rule in Thailand, following a 2006 coup against then-premier Thaksin Shinawatra.

"For Samak, government stability and his political longevity are at risk," said political analyst Thitinan Pongsudhirak of Chulalongkorn University.

"The PAD's protest has sustained for more than three weeks, and they are agitating for blood. They are going for broke every day to bring down the government."

The protests by the so-called People's Alliance for Democracy (PAD), which led rallies against Thaksin before the coup, rarely draw more than a few thousand people.

But they have an outsized influence because the PAD captures the sentiment of Bangkok's elite, who don't trust Samak after he openly campaigned as a proxy for Thaksin.

Thaksin, a self-made billionaire from the northern province of Chiang Mai, alienated much of Thailand's elite, who have traditionally hemmed close to the royal palace or the military.

Both Thaksin and Samak were elected on strong support from Thailand's poor rural heartland, where voters remain loyal to Thaksin after he brought them universal health care, cheap loans and other benefits.

Thitinan says the protest "signifies how power and wealth in Thailand are so lopsided in favour of Bangkok."

"It says that a very small minority can hijack an entire country's democracy," he added.

The protesters want Samak to step down, accusing him of meddling in corruption cases still pending against Thaksin, and of mismanaging the economy.

Unions representing Thailand's 43 state enterprises, as well as truckers, farmers and others, have threatened to take to the streets over economic concerns -- chiefly the soaring price of fuel and food, which last month pushed Thailand's inflation to a 10-year high.

The Democrats' parliamentary motion also accused Samak of mismanaging the economy, but their effort could be tied up in procedural delays for months.

The political turmoil has battered the stock market, which has lost more than 100 points since the protests began, with the Stock Exchange of Thailand dropping again Wednesday to close at 765.74.

Samak inadvertently fuelled the tensions on May 31, when he ordered police and soldiers to clear the protesters from the street where they have camped in Bangkok's government district, sparking fears of bloodshed.

He later backed down, but the protesters seized on the moment as a victory that breathed new life into their campaign.

"The momentum for PAD has been created almost entirely through the actions of the government," said Sunai Phasuk, a researcher for Human Rights Watch.

"With tensions like this, by taking people to the street, it may lead to confrontation, and that could end either as streets clashes ... or a crackdown by government forces."

So far the military says it will not intervene. But that's what the generals said during the protests against Thaksin.