One of Mayor Bloomberg’s Democratic challengers said yesterday that, if elected mayor, he would create a city Homeland Security Office to take over the existing Office of Emergency Management.

The speaker of the City Council, Gifford Miller, said his new director of homeland security would take on all the responsibilities that OEM had under Mayor Giuliani and oversee coordination between city and federal agencies.

The proposal is a direct shot at Mr. Bloomberg, who has diluted the role of OEM since taking office but given more power to police officials when it comes to handling emergencies.

“This mayor has gutted the Office of Emergency Management,” Mr. Miller said in a phone interview. The new director, he said, would act as a “coordinator of existing agencies that are not being coordinated now.”

A spokesman for the mayor, Paul Elliott, said city agencies were working well together and did not need “an added bureaucracy to get in the way of their doing their jobs.”

Mr. Miller began attacking Mr. Bloomberg’s emergency-response plan this month when the city’s highest-ranking fire officer, Chief Thomas Hayden, publicly criticized it.

The plan, which is called the Citywide Incident Management System, sets parameters for when the police and fire departments share command at emergency scenes and when one acts as lead agency. Mr. Hayden took issue with Mr. Bloomberg’s decision to give command to the NYPD when hazardous materials are suspected. Mr. Miller and many of his colleagues in the council adopted that position.

Mr. Miller also took issue with the Bloomberg plan for failing to put in place a system to resolve disputes when police and fire officials disagree about whom should be in charge. His Homeland Security Office would act as a “tiebreaker” in those cases, he said.

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Mayor Bloomberg picked up another union endorsement yesterday, as the New York City Plumbers and Pipefitters union announced it would back him in his bid for re-election in November. The 5,800-member UA Plumbers Local Union No. 1 backed Mr. Bloomberg’s Democratic opponent, Mark Green, in 2001.

Union leaders said Mr. Bloomberg’s record on the economy, crime, and education were the main reasons for the endorsement.

“All the good things this mayor has done have not gone unnoticed, and the members and officers of Local No. 1 wholeheartedly endorse him and stand willing and able to assist his re-election in any way we can,” the union’s business manager, George Reilly, said in a written statement yesterday.