Thai politics has entered a tumultuous new era, analysts say, as the beleaguered ruling party slugs it out against an onslaught of opposition challenges and major rival protests menace the capital.
Tens of thousands of anti-government demonstrators and the pro-government "Red Shirts" are expected to mass in Bangkok Sunday as the turbulent nation sees its most significant political street action since bloody rallies in 2010.
The ruling Puea Thai party -- viewed as a conduit for divisive former premier Thaksin Shinawatra -- is under fire after failing to push through two major policies that have incensed its enemies, and even alienated some supporters.
"We are going to see ongoing tension," said analyst Thitinan Pongsudhirak, of Bangkok's Chulalongkorn University, following a Constitutional Court ruling this week scuppering plans by Prime Minister Yingluck Shinawatra -- Thaksin's sister -- for a fully-elected senate.
"The anti-government protests are strong enough now to stymie government policy work and without the ability to get policy work done, I think that we are beginning to see the new electoral cycle in motion," Thitinan said.
Thailand has been rocked by periodic outbreaks of protest violence since Thaksin was deposed in a military coup seven years ago.
Observers say the latest crisis was triggered by a wave of anger unleashed by a Puea Thai amnesty plan that could have allowed Thaksin's return from self-imposed exile -- and pardoned those responsible for a deadly 2010 military crackdown on his "Red Shirt" supporters.
The plan galvanised the opposition, sparking anti-government protests that have not ebbed despite a senate move quashing the bill last week.
Puea Thai was battered further by the Constitutional Court verdict -- although it escaped the judicial dissolution that removed two previous Thaksin-allied governments.
Now the opposition Democrat Party, whose last stint in power came as a result of a parliamentary vote after those party dissolutions, has lined up a battery of challenges to the government.
Yingluck faces opposition calls to resign and is also set to defend government policy in a no-confidence debate next week.
Other legal and institutional moves include a complaint to the Constitutional Court over a $69 billion Puea Thai infrastructure plan.
"(The court decision) represents the royalist establishment's overarching strategy to push the Yingluck government from power through the use of legal attrition -- wearing away the Puea Thai government bit by bit," said Paul Chambers, an academic at Chiang Mai University.
Thailand, which has seen 18 actual or attempted coups since it became a constitutional monarchy in 1932, has appeared irreconcilably polarised over Thaksin.
The telecoms tycoon-turned-politician has ardent support from many of the country's rural and working class and Red Shirts demonstrators.
But he is loathed among the elite and middle classes, who accuse him of corruption.
He is seen as pitted against the military-backed establishment behind the coup and the royalist and nationalist "Yellow Shirt" protest group, whose street protests helped unseat him.
While the elite-backed Democrats have not won an elected majority in some two decades, Thaksin and his allies have repeatedly demonstrated strong poll support.
Observers say the ruling against the attempted senate amendment was aimed at preventing Thaksin from consolidating power in both houses.
Should the crisis force Yingluck to dissolve parliament's lower house, bringing forward an election currently expected in 2015, her party is expected to win.
"Thaksin simply has the most supporters in Thailand who will vote. But this is not a good time for a dissolution. Now, Puea Thai is the least popular than it ever has been," Chambers said.
The amnesty debacle reduced the party's standing in the eyes of many Reds, who fear it would have absolved key Democrats of blame over the breakup of their 2010 street protests that left more than 90 people dead.
Experts said this is just one of the old allegiances that may be shifting, as power-brokers lay the ground for the future of the country, where the politically-stabilising king is frail and about to turn 86.
Michael Montesano at the Institute of Southeast Asian Studies in Singapore suggested the amnesty bill was a political olive branch between Thaksin and some erstwhile foes, with "the backing of a number of elements of the Thai elite".
But he said the anti-government protests and Red Shirt anger at the amnesty showed these groups had failed to "ensure the buy-in of their mass followings".
"Behind-the-scenes elite fixes no longer work as well as they once did in Thailand," he said.