"I extend my support to Mamata Di on this show of unity and hope that we send a powerful message of a united India together," Gandhi wrote in the letter.
In a Facebook post, he said, "The entire opposition is united in our belief that true nationalism and development can only be defended on the tested pillars of democracy, social justice and secularism, ideas that the BJP and Mr Modi are intent on destroying."
In his letter to Banerjee as well as in the post on the social media platform, Gandhi said there were powerful forces that were fanned by the anger and disappointment of millions of Indians, who had seen through the "false promises and lies peddled by the Modi government".
"These forces are moved by the hope of a new tomorrow; a tomorrow filled with the idea of an India in which the voice of every man, woman and child will be heard and respected no matter what their religion, economic status or region is," he said.
A host of Opposition leaders are likely to attend the Trinamool Congress supremo's rally in Kolkata on Saturday, which is being projected as her "show of opposition strength" ahead of the Lok Sabha polls.