The official, who asked not to be named, said that Pupatello and Takhar have a history together, it was Pupatello who first recruited Takhar into politics many years ago.
He also mentioned that Pupatello stood by Takhar in 2006 when, as a cabinet minister, he was under fire for not keeping an arms-length distance from his business dealings.
Finally, the source said, Takhar and Pupatello have similar policies.
"They both have a small business platform," he said.
The official said that the decision to join Pupatello was made 10 minutes before it was made public. As a result, however, Takhar's name will appear on the second ballot.
Takhar's delegates will give a big boost to Pupatello, who only beat chief rival Kathleen Wynne by two votes in the first ballot.
Earlier in the day, candidate Eric Hoskins was forced to drop out of the race after finishing last on the first ballot. He and most of his delegates have gone to the Wynne camp.
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