Two lawmakers are speaking out against a potential merger between United Airlines and American Airlines, saying a deal would weaken industry competition and harm consumers. 

"If United and American were to merge, airlines would face less pressure from rivals to keep the cost of flying down, and airlines across the industry could raise ticket prices and fees even higher," Sen. Elizabeth Warren, a Democrat from Massachusetts, and Sen. Mike Lee, a Utah Republican, said in an April 19 letter to United CEO Scott Kirby and American CEO Robert Isom.

The letter, which Warren's office shared with CBS News, comes less than a week after Kirby floated the idea of a possible merger between the two carriers.

Combining United and American would also "hurt smaller airlines' ability to compete for critical gate access" and reduce wages for airline workers by reducing the number of employers bidding for their labor, the senators wrote.

In their letter, Warren and Lee also asked the two airlines' chief executives to indicate by May 3 if they have discussed a deal and to detail how a merger could potentially affect American consumers. 

United declined to comment on the lawmakers' letter, while American did not immediately respond to a request for comment. 

Kirby brought up a potential United-American tie-up with Trump administration officials last week, according to sources familiar with the situation. 

But American has dismissed the idea, saying in a statement on Friday that it is "not engaged with or interested in any discussions regarding a merger with United Airlines." 

A merger would be "negative for competition and for consumers," American added.

— CBS News' Kris Van Cleave contributed to this story