(CNN) -- Spirit Airlines isn't happy with the new airline price advertising rule. The big "Warning!" sign popping up on the carrier's website makes that pretty clear.
The message: "New government regulations require us to HIDE taxes in your fares. This is not consumer friendly or in your best interest. It's wrong and you shouldn't stand for it."
The rule, which requires airlines to roll mandatory per-passenger taxes and fees into the advertised fare, went into effect this week as part of a new package of Department of Transportation airline passenger protections. Before the rule, airlines could advertise the base fare or show it on the first screen of online fare results, adding taxes and fees later in the shopping process.
Sen. Barbara Boxer, D-California, called Spirit's message to customers a "deliberate attempt to deceive the flying public" in a letter Thursday to Spirit CEO Ben Baldanza, urging him to take the warning off the airline's website and clarify its message to consumers.
Spirit Airlines is disappointed by the letter, said spokeswoman Misty Pinson. "We would normally expect Senators to encourage (First) Amendment protection," Pinson said via e-mail.
"We have always shown taxes before someone purchased. They now want them hidden. It is wrong and we will fight for consumers," Pinson said. Spirit's online message encourages customers to contact lawmakers to oppose the rule.
Boxer's office isn't buying the airline's First Amendment argument.
"Spirit Airlines seems to think it has a First Amendment right to deliberately mislead its own customers. I'm not sure that's what our founding fathers had in mind," said Boxer spokesman Andy Stone, noting that the Department of Transportation has fined Spirit in the past for deceptive price advertising.
The Business Travel Coalition, a group advocating for transparency, calls Spirit's message "a disgraceful lie" employing "over-the-top fear tactics."
In messages to consumers, Spirit said the government has a "hidden agenda" to increase taxes.
The Obama administration has proposed raising air travel taxes, which the DOT says is unrelated to the new regulation.
"We are simply requiring airlines to post the full fare, and we leave it to them to break it down if they want," DOT spokesman Bill Mosley said.
Spirit is not the only airline that objected to the new regulations. Spirit, Southwest Airlines and Allegiant Air have all made legal appeals to have the price advertising rule overturned.
Southwest said the rule singles out airlines.
"Our main objection ... is that there is no justification for treating air travel differently from just about everything else that consumers purchase, i.e. they pay for the price of goods and services and then pay tax. And that's how everything is advertised, as the price of the item separately from the tax on that price," Southwest spokeswoman Brandy King told CNN last month.
Southwest's message to passengers about the new rule takes a different tack, explaining the changes and assuring consumers that while fares look higher, the airline and subsidiary AirTran have not raised fares.