Ambassador Nikki Haley’s resignation as the top U.S. diplomat will leave a "leadership vacuum" for American foreign policy, according to a top Senate Democrat.

“I am deeply concerned about the leadership vacuum she leaves and the national security impact of her departure at this time of continued disarray for this administration,” New Jersey Sen. Bob Menendez, the ranking member on the Foreign Relations Committee, said Tuesday.

[READ: Nikki Haley's letter of resignation]

Menendez is one of the Trump administration's top foreign policy critics, but he praised Haley for standing up to President Trump.

“I want to thank Ambassador Haley for her willingness to express moral clarity to the world and to President Trump, and promote American values and leadership on the global stage, even when she lacked the backing of the White House or State Department,” Menendez said.

The president was also effusive. “Nikki Haley has been very special to me," Trump said in an on-camera announcement of her departure.

Trump added that he would likely choose her replacement within a few weeks, but Menendez pointed to the open seat at the U.N. as another high-profile vacancy for an administration struggling to fill diplomatic posts.

“This administration’s continued inability to adequately or appropriately fill key national security positions puts our nation at risk,” he said. “The president and secretary of state must immediately name a new ambassador to the United Nations for the Senate’s consideration, and commit to a pragmatic path forward that embraces America’s history of shaping the international institutions that have elevated America’s role in promoting peace, human rights, and security around the world.”

Secretary of State Mike Pompeo has moved aggressively to fill diplomatic posts since replacing former Exxon Mobil CEO Rex Tillerson as the nation’s top diplomat. He accused Menendez of stonewalling nominations last week, after the New Jersey Democrat threatened to hold up nominees over a dispute about the State Department contributing funding for deportations to Mexico.

“We have members of the United States Senate who — for whom partisanship has now driven delay and obstruction of getting America’s diplomatic corps into every corner of the world,” Pompeo said.

Menendez’s team countered Pompeo’s blame misdirected, because he “does not have holds on most of the nominees pending before the Senate” and Trump has failed to nominate diplomats to some major diplomatic posts. “I look forward to working with the State Department to ensure the Senate Foreign Relations Committee fully exercises our power and authority to conduct a thorough, accurate, and vigorous confirmation process for Ambassador Haley’s successor,” the senator said Tuesday.

[Opinion: Here's who Trump should pick to replace Nikki Haley at the UN]