Colorado voters kicked the mayor to the curb at the ballot box again, rejecting a proposed statewide income-tax hike backed by Mr. Bloomberg just two months after ushering out two Democratic state senators who supported his gun-control agenda.

Mr. Bloomberg sunk $1 million into the $10 million campaign for Amendment 66, a proposed $1 billion tax increase to fund K-12 education. Despite an enormous spending advantage — opponents of the ballot measure only raised about $20,000 — Amendment 66 lost by a whopping 66 percent to 34 percent.