Tim Miller, a Bush spokesman, confirmed Kyrillos’s support Monday as well as a $10,000 donation Kyrillos made in March to Bush’s political-action committee.

The defection of Kyrillos to Bush’s camp is a blow to Christie, who places a premium on loyalty and had hoped to keep his network of allies in New Jersey with him, should he decide to seek the Republican nomination.

Not only is Christie losing a confidant, but a plugged-in operative and former state party chairman who is close with New Jersey’s biggest donors. In 2008, Kyrillos chaired Mitt Romney’s presidential campaign in the state.

Kyrillos is one of several influential Christie associates whose ties to the governor have frayed in recent months. Many power brokers who once cheered his rise are now either hesitant to back Christie as he eyes a campaign for the White House, or shifting allegiances.