WASHINGTON — The Senate’s top Democrat on Monday asked the Federal Communications Commission to consider whether two major Chinese telecommunications companies should be barred from operating in the United States.

Senator Chuck Schumer of New York, the Democratic leader, along with Senator Tom Cotton, an Arkansas Republican, cited national security concerns in a letter asking the commission to review the licenses that give the two companies, China Telecom and China Unicom, the right to use networks in the United States.

In the letter, they said that the two Chinese government-linked telecom operators could use that access to “target” Americans’ communications. And they warned that the companies could reroute communications traveling on their networks through China. The text of the letter was obtained by The New York Times.

The request by the two senators shows how leaders in Congress are willing to apply more pressure to Chinese technology firms even as President Trump seems open to pulling back on some of the restrictions as part of the trade negotiations with China. The focus on two previously untouched firms shows that efforts to restrict Chinese technology firms are expanding.

Brian Hart, an F.C.C. spokesman, said that Ajit Pai, the F.C.C. chairman, had made it clear that the agency was “reviewing other Chinese communications companies such as China Telecom and China Unicom” but didn’t commit to opening a formal proceeding to look at the licenses.

China Telecom denied that it represents a national security threat to the United States.

“We make the protection of our customers’ data a priority, and have built a solid reputation as one of the best telecom companies in the world,” said Ge Yu, a spokesman for China Telecom’s Americas subsidiary, adding that the company is proud of “maintaining a good standing with all regulatory agencies.”

In recent years, Washington has levied $360 billion in tariffs on Chinese goods, moved to block the ability of American companies to export their technology to China and tried to stop certain products linked to the Chinese government from being sold in America.

National security officials have been worried for years that the Chinese government could use its companies to gain access to crucial telecommunications infrastructure. Those concerns have become more prominent as carriers in the United States and in China race to launch next-generation 5G wireless networks — and American regulators have targeted Chinese telecom companies in the name of security.

In May the F.C.C. denied an application from China Mobile to operate in the United States. Ajit Pai, the commission’s chairman, said at the time that there was a risk that the Chinese state would use the carrier to “conduct activities that would seriously jeopardize the national security, law enforcement, and economic interests of the United States.”

But China Unicom and China Telecom have retained the ability to operate in the United States. Both have licenses that were granted by the F.C.C. in the early 2000s, and some regulators have said that they should be re-examined even though the companies are smaller than China Mobile.

Brendan Carr, a Republican commissioner on the F.C.C., said in May that the agency should go “even further” than denying China Mobile’s application and look at the other two companies. A Democrat on the commission, Geoffrey Starks, said that the “national security environment has changed” since the firms were allowed access to networks in the United States.

Mr. Schumer and Mr. Cotton echoed those concerns in their letter, saying that “the evolving national security environment and increased knowledge of the Chinese government’s role in economic and other forms of espionage” requires a re-examination of licenses belonging to any state-controlled Chinese telecom company

Some experts disagreed with the idea that cutting off China Unicom and China Telecom would protect national security, in part because they largely serve companies that do business in China as well as some Chinese consumers in the United States.

“They’re not operating in the sense of a Bell company, or a T-Mobile, or a Verizon; they don’t compete in the U.S.,” said James Lewis, a senior vice president at the Center for Strategic and International Studies. “I think it’s overblown.”

Revoking the licenses would be the latest way the United States has tried to check China’s tech sector since Mr. Trump’s 2017 inauguration.

The Committee on Foreign Investment in the United States, the panel that vets deals between American and foreign firms, said in 2018 that the acquisition of the chip maker Qualcomm by then Singapore-based Broadcom could limit America’s ability to compete. The same panel demanded this year that a Chinese gaming company sell the gay dating app Grindr.

The administration has been especially aggressive in cutting off Chinese telecom companies’ access to American customers. It banned the networking manufacturer ZTE from buying American products in 2018. Earlier this year, it placed Huawei on a Commerce Department list of companies to which American companies cannot export goods.

Some of the Huawei restrictions are expected to take effect in November, but the White House has reconsidered its Chinese telecom restrictions under pressure from American companies and the Chinese government during on-again, off-again trade negotiations.

After China’s president, Xi Jinping, appealed on the company’s behalf, Mr. Trump interceded with his administration to reverse the ZTE ban. In trade negotiations this summer, the administration appeared willing to back down from some of the restrictions it had placed on Huawei.

Lawmakers have been taking their own steps to put pressure on the companies.

Senator Mitch McConnell of Kentucky, the Republican leader, has expressed interest in expediting consideration of a resolution declaring that Chinese telecom companies like Huawei and ZTE pose a national security threat to the United States, according to a congressional aide with direct knowledge of the matter.

Such requests can be blocked if a senator objects. A spokesman for Mr. McConnell, David Popp, declined to comment.

But a nonbinding resolution is unlikely to satisfy lawmakers in both chambers, including Mr. Schumer, who are trying to make sure that language giving Congress more oversight over some of the Huawei restrictions is included in the upcoming military spending bill.

“If we’re serious about this, we need to have something with teeth,” said Senator Chris Van Hollen, a Democrat from Maryland. “Making a statement may have symbolic importance but it does nothing to address the real issues.”

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