Warning that there is no fallback plan to fund a State Police presence in the French Quarter if voters there reject a new sales tax, Mayor Mitch Landrieu on Thursday urged those voters to support the measure Saturday.

Without the tax, “we have to go back to the drawing board,” Landrieu said at a news conference outside the 8th District police station on Royal Street.

Only residents in the Quarter will vote on the quarter-cent sales tax increase Saturday. The tax would amount to a 25-cent increase in the cost of a $100 purchase. Most of the burden would fall on visitors, though hotel stays would be exempt.

“You cannot have something for nothing,” Landrieu said. “You have to come up with a way to pay.”

The increased State Police presence in the French Quarter, which has been ongoing throughout the year, would be paid for through the tax as part of a general plan to beef up law enforcement in the district after a series of high-profile crimes.

The tax is expected to generate about $2 million a year. That money would be combined with another $3 million, much of which will come from the hospitality industry, for the next five years to pay for the State Police presence in the Quarter.

NOPD Superintendent Michael Harrison said crime in the Quarter has fallen by 15 percent in the six months that troopers have been on patrol; violent crimes have fallen by 35 percent.

Groups representing French Quarter residents and businesses, the hospitality industry and the City Council all have endorsed the measure.

Under the deal, at least 30 troopers would be assigned to the French Quarter. At present, between 15 and 40 troopers have that assignment, depending on the day.

“The public could care less about the color of our uniforms or the shape of our badges,” State Police Superintendent Col. Mike Edmonson said. “They want to know when they call us that we’re coming.”

Follow Jeff Adelson on Twitter, @jadelson.