ATLANTA — Six Democratic senators want to hit U.S. airlines with a tax if they charge passengers for their carry-on bags.

The senators said Wednesday that this would keep more airlines from following Spirit Airlines’ lead. The small Florida airline said last week that starting Aug. 1 it will charge its customers as much as $45 to bring a bag aboard its aircraft and put it in an overhead bin.

Air travelers have been forced to pay a barrage of fees for once-free amenities since 2008, for everything from checked bags to pillows to food. That has not stopped them from flying, but critics say charging for carry-on bags is stepping over the line.

The senators — Jeanne Shaheen of New Hampshire, Charles Schumer of New York, Ben Cardin of Maryland, Amy Klobuchar of Minnesota and Robert Menendez and Frank Lautenberg of New Jersey — want to designate carry-on baggage as a necessity.

Airlines currently pay a 7.5-cent tax to the federal government for every dollar they collect in fares, but no tax is imposed on fees collected for nonessential services, the senators said.

“As a result, airlines can impose fees on these bags without paying any tax to the federal government on the revenues they collect,” the senators said.

Add-on fees have been an important source of revenue for airlines since oil prices soared to an all-time high of $147 a barrel in July 2008. Those fees became a mainstay for airlines’ bottom lines even after fuel prices plunged.

It’s unclear if the anti-carry-on fees legislation will gain traction among Republicans and win support in Congress.