Sen. Chris Murphy, a Democrat on the Senate Foreign Relations Committee, called Friday for a probe into Rudy Giuliani's efforts to influence investigations in Ukraine he anticipates as possibly beneficial to President Donald Trump, citing "the implications of this for United States foreign policy."

In a letter to the committee's Republican chairman, Murphy, D-Conn., said that he was "alarmed" after reading reports that Giuliani, Trump's personal attorney, had planned to travel to the country to push Volodymyr Zelensky, the Ukrainian president-elect, to move ahead with investigations involving former Vice President Joe Biden's son, Hunter Biden, and probes related to special counsel Robert Mueller's Russia investigation.

"I am deeply concerned about the implications of this for United States foreign policy. Specifically, Rudolph Giuliani, the President’s personal lawyer, has apparently held meetings with Ukrainian officials in the United States and plans to travel to Ukraine for further discussions," Murphy wrote in the letter addressed to Sen. Jim Risch, R-Idaho, the committee's chair.

"As far as we know, none of these meetings are being coordinated with the U.S. State Department or other government agencies," Murphy wrote.

"What’s he worried about?" Giuliani told NBC News in a text message in response to Murphy's call for an investigation. "It will show his party conspired with Ukrainian officials to affect the election. He has no idea what he’s opening up. How smart is he? I don’t know him."

Giuliani said Friday night that he was no longer planning to travel to Ukraine.

Risch did not immediately respond to a request for comment from NBC News. Biden has emerged as an early front-runner in a crowded field of Democrats angling to take on Trump in 2020.

Giuliani told The New York Times in an interview published Thursday that "we're not meddling in an election, we're meddling in an investigation, which we have a right to do," adding that "there's nothing illegal about it" even if some people "say it's improper."

Giuliani told The Times Thursday that Trump fully supports his plans.

Murphy, however, said the move raises a series of red flags. He wrote in the letter that "these actions are entirely improper and could be illegal."

Murphy added that he wants to know if the State Department has been briefed on the former New York City mayor's efforts, if Giuliani's "private foreign policy engagement" resulted in any official foreign policy decisions and if Giuliani has had any access to classified information and diplomatic cables, among other subjects.

"The Senate Foreign Relations Committee should be concerned that the President of the United States is making foreign policy decisions through personal or campaign representatives that do not go through the Department of State, preventing us from exercising our constitutional oversight responsibilities on behalf of the American people," Murphy said.

"Further, the possibility that a personal representative of the President is engaging with foreign governments in order to obtain personal or political gain is a matter that must be thoroughly examined."