Commissioners approved an ordinance Tuesday for a March 12 election.
If voters strike down the tax, the county and the cities will be hard pressed to find the more than $7 million per year it costs to fund the programs, County Executive Office Kim Carpenter said.
He said the county and Farmington would each have to pay 44 percent of the bill and Bloomfield and Aztec would each pay 6 percent of the bill if voters turn down the tax.
"It is very important people know this is already an existing tax and it is extremely important to not only the county, but to the fiscal operations of all cities as well," he said.
The gross receipt tax has been in place since 2003 but will expire in June because of a "sunset" provision. The election will determine if the tax will continue indefinitely, San Juan County Attorney Jim Durrett said.
The tax -- 0.18 percent of gross receipts collected in San Juan County -- pays for the countywide emergency dispatch center and ambulance services.
Carpenter said the tax is the sole funding source for those emergency response programs.
In the 2012 fiscal year, the tax amounted to $7.3 million. The most ever accumulated was in the 2009 fiscal year, when it amounted to $8.3 million, said Marcella Brashear, the county's chief financial officer.