''Given the overwhelming impact and implications of moving the UDB line, it would be irresponsible for us, as public servants, to take action without first establishing a comprehensive strategy for the inevitable future expansion of the county,'' Alvarez wrote in a message attached to the vetoes, which he issued early Wednesday.

The County Commission could try to override the veto at its Dec. 18 meeting, but one of the five opponents would need to change sides in order to reach the two-thirds margin. The five no votes were Carlos Gimenez, Sally Heyman, Dennis Moss, Katy Sorenson and Rebeca Sosa.

Without a veto, the three applications outside the development boundary would likely die until 2009 because -- changes to the line are only considered during one brief window every two years. The other nine applications could be reconsidered in about six months.

"I strongly urge the board to sustain this veto and work, utilizing all available resources, to create and adopt a comprehensive and coherent vision for the future development of Miami-Dade County.''

The development boundary is an imaginary line created to limit urban sprawl and protect the environment; beyond the line, high-density development is largely forbidden.

The commission's Nov. 27 vote was only a preliminary step, sending the applications for further review to the state's Department of Community Affairs before a final vote in April. But Alvarez said even that action "initiates a move in a dangerous direction without compelling arguments to warrant the move.''

In his message, Alvarez said he was legally unable to just strike down the projects outside the UDB because all 12 applications were part of the same resolution approved by the commission.

''I am vetoing the entire package not to disparage the merits of each one individually, but express my concern with those that could jeopardize the livelihood and sustainability of our county for future generations,'' he wrote.