Congress President Sonia Gandhi congratulated Shiv Sena Chief Uddhav Thackeray ahead of his swearing-in ceremony and expressed regret that she won't be able to attend the event on Thursday evening in Mumbai.
Uddhav's son Aaditya had met Sonia Wednesday night and extended the former's invite to the Congress president.
In her letter, Sonia wrote that the Sena, Congress and NCP trio had come together under "quite extraordinary circumstances, at a time when the country faces unprecedented threats from BJP."
She wrote that the political atmosphere had become "poisonous", the "economy had collapsed" and farmers were facing "great distress".
The three parties, she said, have agreed to a common minimum programme and hoped everyone will work towards ensuring it's implemented in letter and spirit.
The Raebareli MP also said that the people of Maharashtra expected the alliance to provide a "cohesive, purposeful and responsible" administration.
Uddhav will be the first from the Thackeray family to be sworn in as the chief minister of the state. At least, six ministers, two each from Sena, Congress and NCP, are expected to take oath alongside Uddhav.