Some Democratic lawmakers quickly questioned the merits of the deal. Senator Amy Klobuchar of Minnesota said in a statement, “I remain concerned that increased consolidation could undermine benefits to consumers.” But Gov. Jay Inslee, a Democrat from T-Mobile’s home state of Washington, tweeted that he welcomed the news of potential job creation.

T-Mobile and Sprint contend that the market looks different than it did when they last tried to combine in 2014. The need to build out 5G requires tens of billions of dollars in investments that T-Mobile and Sprint, especially, would be hard-pressed to put up on their own. And there are fresh competitors in the sector. Comcast, for example, has quickly drawn hundreds of thousands of wireless service customers by bundling mobile phones with their cable plans, even if it loses the company money.

A huge part of T-Mobile and Sprint’s push is emphasizing the future of 5G. Proponents say the superfast wireless standard would not only make downloading movies faster, but underpin huge advances in autonomous vehicles, internet-connected devices and more.

The White House has declared 5G wireless a vital national priority. In March, the Trump administration blocked a hostile bid by Singapore-based Broadcom for San Diego-based Qualcomm, citing national security concerns. Some analysts questioned whether the predominantly foreign ownership of the combined company — including SoftBank, which has business ties to Chinese companies like Huawei — poses possible national security risks.

Investing in new 5G networks as separate companies would be difficult for T-Mobile and Sprint, given their current financial situations. Sprint has about $32 billion in debt on its books, while T-Mobile generates a fraction of the cash that Verizon and AT&T do. But combining would yield some $6 billion in cost savings, especially because the companies would need to pay to run just one network instead of two, allowing them to spend more money on infrastructure.

“We can change the nature of the curve in the way the U.S. and others are investing in 5G,” Mr. Legere said in the interview. He added that he shared worries “that the U.S. is on the verge of losing the leadership it has attained” in 5G.

For consumer advocates, however, the chief worry is that a shrinking number of providers would bring an end to the innovations that T-Mobile introduced to the American wireless market since the Justice Department blocked its plan to sell itself to AT&T in 2011. Under Mr. Legere, T-Mobile cut prices, ended long-term contract requirements and promised to simplify customer bills by eliminating hidden fees and surprise taxes.