Democrat Sen. Richard Blumenthal said a proposed partnership between United Airlines and smaller JetBlue Airways could harm competition and lead to fewer and more expensive options for consumers.

In a letter seen by Reuters, Blumenthal asked United CEO Scott Kirby and JetBlue CEO Joanna Geraghty to answer questions about their "Blue Sky" tie-up to allows travelers to book flights on both carriers' websites, while interchangeably earning and using points in their frequent flyer programs.

Blumenthal asked the airlines to disclose records about the partnership and future plans, adding he is concerned about any deal "that may harm full and fair airline competition and lead to fewer and more expensive options for travelers, particularly in the New York City area."