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WASHINGTON (Reuters) - Senators Amy Klobuchar, Elizabeth Warren and other Democratic lawmakers expressed "serious concerns" on Monday about T-Mobile US, Inc's DTEGn.DE plan to buy rival Sprint Corp S.N, focusing on the planned deal's effect on lower-cost wireless plans, Klobuchar's office said in a press statement.

In a letter sent to the Justice Department’s antitrust chief, Makan Delrahim, and Federal Communications Commission Chairman Ajit Pai, the senators said that they worried that the deal between the No. 3 and No. 4 wireless service providers would lead to higher prices for consumers.

“T-Mobile and Sprint have led the way in offering wireless products and service options that are more appealing to lower-income consumers, including no contract plans, prepaid and no credit check plans, and unlimited text, voice, and data plans,” the lawmakers wrote.

While AT&T and Verizon dominate the U.S. wireless market overall, T-Mobile is the most popular among customers who make less than $75,000 per year, and Sprint’s prepaid brand Boost counts 83 percent of its users in that income range, according to data from Kagan, S&P Global Market Intelligence data.

T-Mobile has 38 percent of the U.S. prepaid market, while Sprint has 16 percent, which would give the combined company 54 percent, according to S&P.

In addition to Klobuchar and Warren, the letter was signed by Democratic Senators Richard Blumenthal, Mazie Hirono, Tina Smith, Tom Udall and Ed Markey, according to the press statement.

Also signing was former presidential candidate Bernie Sanders, an independent.