Thaksin Shinawatra called the election three years early in an attempt to end weeks of political turmoil set off by his family's $1.9 billion tax-free sale of stock in the telecoms firm he founded before entering politics.
Thousands of protesters gathered on Thursday in Bangkok to demand that the election be abandoned altogether - a continuation of the regular street rallies that have dogged Thaksin's administration since February.
And protest organisers have vowed to continue their campaign despite the outcome of the vote, which they say will give Thaksin's ruling Thai Rak Thai (Thais Love Thais) party unchallenged power.
Thai Rak Thai is expected to run unopposed in about 168 of 400 constituencies but dozens of the party's candidates could fail to win the 20% of eligible votes required to take office.
About a third of some 941 candidates have already been disqualified, meaning that there might not be enough legislators elected to confirm a new prime minister, forcing by-elections and bogging the country down in months of political limbo.
Thaksin insists that he will survive the poll but has said he will not to take office if he wins less than 50% of the vote.
"Those who boycott, those who don't like me ... they will not accept me anyway," Thaksin said in an interview with the BBC.
While his autocratic style of governance has angered many in Thailand's urban middle class, he remains hugely popular in the rural areas where 60% of the country's 63 million people live.
There is also evidence that support for the protesters appears to be waning.
Opinion polls show Bangkok is wearying of the constant demonstrations and the disruptions to normal life they cause.
In its latest poll, Assumption University said 70% of its 3,606 respondents in and around Bangkok wanted the street protests to stop.
It also said the proportion of people who now say they will vote had leapt to 74% from 41.4% on March 25.