Washington (CNN) Secretary of State Rex Tillerson is moving to eliminate or downgrade special envoy positions at the State Department, including the representative for climate change, a step that is sure to ignite vociferous opposition from some members of Congress.

In a letter obtained by CNN and written to Senator Bob Corker, the Tennessee Republican who heads the foreign relations committee, Tillerson said he would end or transfer as many as three dozen special envoy positions.

"I believe that the Department will be able to better execute its mission by integrating certain envoys and special representative offices within the regional and functional bureaus," Tillerson wrote Corker, "and eliminating those that have accomplished or outlived their original purpose."

Special envoys for Syria, Sudan and South Sudan, and the Arctic will be eliminated.

Cuts and one area of expansion

Other positions dedicated to thorny diplomatic issues, ranging from Mideast peace to relations with Afghanistan, would be subsumed under existing State Department bureaus. One area of oversight -- the Office of Global Food Security -- would be moved to USAID.

Three offices would be expanded -- those dealing with religious freedom, HIV/AIDS and Holocaust issues. The special envoy for Israeli-Palestinian negotiations would remain under the office of the secretary.

Tillerson hinted at a reorganization on his first day at the State Department, even before the Trump administration signaled its intent to cut up to 30 percent of the agency's budget. The letter to Corker outlines some of the first concrete steps the former ExxonMobil CEO will take in his restructuring.

Tillerson notes that the department has nearly 70 special envoys, many that still exist even though the underlying issues have been resolved. But his critics, who pushed back against the proposals at a July hearing on Capitol Hill, pointed out that a special envoy can focus attention on issues important to US national security interests that might otherwise get overlooked.

Indeed, Tillerson himself appointed a special representative for Ukraine negotiations, career diplomat Kurt Volker, in July. In that position, Volker will play a lead role in working on a peace agreement between Russia and Ukraine -- a crucial step to achieving President Donald Trump's goal of better relations with Russia.

At the hearing, as in the letter, Tillerson argued that the State Department "will be able to better execute its mission by integrating certain envoys and special representative offices within the regional and functional bureaus, and eliminating those that have accomplished or outlived their original purpose."

The revamp will "eliminate redundancies that dilute the ability of a bureau to deliver on its primary functions," Tillerson said. These changes, he said, will advance US national security interests and will help to "counter the influence of US adversaries and competitors."

Tillerson has gotten the backing of some think tanks and policy groups in Washington that had been recommending a review of the envoys. In 2014, the American Foreign Service Association said that the number of special envoys and representatives had "increased substantially, diluting the brand and reducing effectiveness."

A State Department official told CNN after this story was first published that Tillerson believes integrating the functions of certain envoys into the bureau structures will make the agency more efficient.

The official, speaking on condition of anonymity to discuss State Department business, said that of 66 positions outlined in the letter, Tillerson will keep 30 envoys and representatives with their title. The work of another 21 will be integrated into bureaus, nine positions will be eliminated entirely and another five will be folded into existing positions.

Bureaus will take over the envoys' allocated budgets, staff and responsibilities. Senior officials will be empowered to conduct high-level diplomacy with foreign officials on issues that call for it.

"Empowering regional and functional bureaus will make knowledge and resources more accessible, provide clarity in reporting authority, strengthen communication channels and create a more efficient State Department," the official said in an email.

Corker expressed support for Tillerson's move on Monday, noting that the State Department reauthorization bill his committee passed directed Foggy Bottom to tell Congress which special envoys it wanted to keep in place.

"Through the years, numbers of special envoys have accumulated at the State Department, and in many cases, their creation has done more harm than good by creating an environment in which people work around the normal diplomatic processes in lieu of streamlining them," Corker said in a statement to CNN. " I appreciate the work Secretary Tillerson has done to responsibly review the organizational structure of special envoys and look forward to going through these changes in detail."

At the testy Senate foreign relations hearing in July, Democrats expressed concern about eliminating special envoy positions.

"I'm open on this because I agree with the chairman, we have too many special envoys," said Maryland Sen. Ben Cardin, the top Democrat on the committee. "On the other hand, there are areas that I want to have special attention, where I don't think you get it unless there's a point person within the State Department to deal with it, and I don't have that comfort level as to how we're going to resolve this."

Sen. Jeanne Shaheen, a New Hampshire Democrat, pushed to get language in the Senate's State Department reauthorization bill this year that would preserve the ambassador-at-large for global women's issues.

Tillerson said in his letter that position will continue to be a Senate confirmed ambassador-level position, which will report to the undersecretary for civilian security, democracy and human rights.

Sen. Ed Markey, a Massachusetts Democrat, took issue with the idea of eliminating several specific positions slated to be axed, including the special envoy for climate change, the coordinator for sanctions policy, and the special representative for Northern Ireland issues.

"None of these are incidental," Markey told Tillerson. "Each one of these areas has a reason why they have a special envoy."

"Slipping through the cracks"

"If they're moved into kind of larger parts of the agency that don't have any squarely aligned responsibility and with a senior person inside the Department, it just would run the risk of slipping through the cracks, of not getting the attention needed," Markey said.

The functions and staff of the special envoy for North Korean human rights issues would now fall under the office of the under secretary for civilian security, democracy and human rights, who will now also assume that title. The position of special envoy for the six-party talks dealing with North Korea will be removed, as the talks ended in 2008.

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The positions of the special envoy for climate change and the special representative for the Arctic region will be removed and their functions and staff placed under the Bureau of Oceans and International and Scientific Affairs.

The special envoys for the Great Lakes region of Africa and Democratic Republic of Congo, and for Sudan and South Sudan will be subsumed under the Bureau of African Affairs.

There will no longer be a special representative and policy coordinator for Burma, with that work and any staff going to the Bureau of East Asian and Pacific Affairs.

And the Bureau of Near East Affairs will take over the work and staff of the special coordinator for Libya. That bureau will keep the functions of a special envoy for Syria, but the title will fall by the wayside and the work will be done by one of the bureau's deputy assistant secretaries.

Update: This story has been updated to include Corker's Monday statement.

Correction: This story has been corrected to reflect that the special envoy for Israeli-Palestinian negotiations would remain under the office of the secretary.