Singapore Democratic Alliance chief Desmond Lim said on Wednesday morning that he disagreed with comments from Workers' Party leader Low Thia Khiang that opposition unity is "inconceivable".
Speaking to reporters during his walkabout, Mr Lim said it is possible to bring different parties, with different personalities and philosophies, together.
"It is about compromise, give-and-take, and finding a common ground among everyone," said Mr Lim, who cited the SDA as an example of how opposition unity can be achieved.
The alliance, which was founded in 2001, now comprises the Singapore Justice party and the Singapore Malay National Organisation.
Mr Lim, 45, said the Reform Party's offer to let him speak for 20 minutes at its rally on Thursday was another example of how parties can work together.
Mr Low said at WP's rally on Tuesday night that the diversity in the "complex" opposition camp, with its different leaders and political beliefs, makes it unimaginable for unity to be achieved.
He added that forcing different personalities together may lead to ugly fights, a scenario that will cause people to lose confidence in the opposition.
Mr Lim said that bringing parties together is difficult for now, as "there are parties that are not interested".
He said that if elected as an MP, he will talk to the different parties and urge them to work more closely together.