The surprise announcement ended speculation he would endorse Republican candidate Joseph Lhota after the strong showing in Tuesday's Democratic primary victory by city public advocate Bill de Blasio, a fierce critic of Bloomberg's policies, The New York Times said.
It is not yet clear if de Blasio received the required 40 percent of votes to win the nomination outright and avoid a runoff .
"Helping the next mayor get prepared for the job ... is really one of the most important things I can do for New Yorkers after November," Bloomberg, a three-term mayor, said. "I don't want to do anything that complicates it for the next mayor, and that's one of the reason I've decided I'm just not going to make an endorsement in the race."
The newspaper pointed out Bloomberg has a rocky relationship with both de Blasio and Lhota, but the decision could be seen as a blow to Lhota, who has said he would seek the support of Bloomberg.
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