"I felt neglected within BJP on a number of occasions and wanted to return home (Congress)," he said.
Kandari was unhappy with the BJP leadership for not nominating him from Tehri LS seat and inducting his detractor and Congress leader from Rudraprayag Virendra Bisht into the party.
He had sent his resignation to state party chief Bishan Singh Chufal three days ago triggering speculation that he might switch loyalties.
Kandari, who joined Congress at an impressive rally here, said, "I returned to Congress as I realised that the party (BJP) I served for over two decades was not taking collective decisions and violating democratic norms."
A former BJP MLA in Uttar Pradesh and Uttarakhand for 12 consecutive years, he was a cabinet minister in the BJP government led by Ramesh Pokhriyal Nishank in the hill state.
Welcoming him at the rally held to drum up support for Saket Bahuguna, party candidate and son of Chief Minister Vijay Bahuguna, AICC incharge of party affairs in the state Chaudhry Virendra Singh said Kandari's entry will brighten prospects of the ruling party.
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Singh said desertion of BJP by senior leaders like Kandari reflected their disillusionment with the saffron party.
State BJP leaders, however, downplayed the issue of Kandari's unhappiness. The party leadership had been trying to mollify him for the last few days and dissuade him from quitting the party.
However, efforts by the BJP to placate Kandari failed as he joined Congress along with scores of his supporters at the rally.
Clarifying that the decision to join Congress was his own, Kandari said, "I approached the Congress myself and told them that I wanted to come back home."
The bye-election, slated for October 10, was necessitated as the Chief Minister vacated the seat on his victory from Sitarganj Assembly constituency. BJP has fielded Maharani Raj Lakshmi Shah, a member of the erstwhile Tehri royal family, setting the stage for a razor sharp contest.