Public gatherings are set to be held across the country Saturday -- including in the Chiang Mai hometown of divisive billionaire ex-premier Thaksin Shinawatra.
Thaksin, who lives in self-exile to avoid jail in Thailand, is adored by rice farmers and large sections of urban working-class voters for his pro-poor policies and steering of the economy.
But he is despised by the royalist elite, whose allies have failed to beat him at the polls since 2001 and instead have relied on coups and court rulings to topple Shinawatra-backed governments.
Thousands of people massed in Bangkok's historic centre late Friday for a defiant campaign by Thai Raksa Chart, one of several parties linked to the Shinawatra clan.
But Thai Raksa Chart could be dissolved by the Consitutional Court on March 7, after its bid to run a princess as candidate for prime minister spectacularly unravelled.
Supporters say the court has the future of Thai democracy in its hands -- with the junta determined to return as a civilian government.
"If it is dissolved it will damage our hopes for democracy badly," said Chailerm Phothijad, 55, at Friday night's rally in Bangkok.
Thailand, which has been stuck on a carousel of coups, violent protests and short-lived civilian governments since 2006 when the army booted Thaksin from office, remains deeply polarised.
Nerves are mounting inside Thai Raksa Chart as the crucial ruling nears. But the party "will continue to campaign as we have been... and leave the issue of judiciary to the court," said Umesh Pandey, a Thai Raksa Chart party list candidate.
Thailand's contentious junta-drafted constitution allows the junta to appoint the 250-member senate, whose votes will count towards choosing the next prime minister.
There are 500 lower house seats up for grabs, 350 through constituencies and the remainder through the party list -- a system of proportional representation meant to give smaller parties a foothold.
Junta leader Prayut Chan-O-Cha, who took power in a 2014 coup, wants to return as prime minister. With Thai Raksa Chart's political fate in the balance, another anti-junta party led by a billionaire is emerging as an untested -- but significant -- force.
Future Forward, led by Thanathorn Juangroongruangkit, the 40-year-old scion of Thailand's biggest auto-parts maker, has won favour among the kingdom's millennials.
More than seven million people aged between 18 and 25 are eligible to vote this month for the first time, according to the Interior Ministry.
Analysts say if Thai Raksa Chart is disbanded its supporters may cast votes for Future Forward.