The letter, addressed to the citizens of Delhi, ended with this appeal: "I have faith in you. You will give a fitting and effective reply."
AAP has challenged the disqualification of its legislators from the Delhi assembly by President Ram Nath Kovind, on the recommendation of the Election Commission.
"Is it justified to remove elected lawmakers in this unconstitutional and illegal manner? Isn't it dirty politics?" Mr Sisodia questioned in the letter in Hindi that critics said looked like AAP's early pitch for by-polls that seem inevitable.
Mr Sisodia targeted the BJP government at the centre. The letter gave a detailed explanation on the legislators who had been parliamentary secretaries between March 2015 and September 2016.
"We had given these legislators different responsibilities...like looking after schools or hospitals...they were not given any official car or house, and not even a paisa as salary...How is it an office of profit then?" Mr Sisodia said.
Accusing the BJP of "trying every trick" to harass the AAP government in its three years in power, he referred to the criminal cases against AAP MLAs and the CBI raid at Chief Minister Arvind Kejriwal's office in 2015.
AAP's number two leader detailed what his government had achieved in the past three years, and plans for the next two years. He alleged that the centre's strategy was to keep Delhi in election mode this year as well as the next -- when national polls are due -- so that the AAP government would be unable to do its job because of the model code of conduct.
"They are worried, scared about the speed at which the Kejriwal government is working and Kejriwal's growing popularity."
AAP leaders had requested President Kovind to hear their version before deciding on the Election Commission's recommendation, but he signed off on disqualifying the party's MLAs yesterday.