WASHINGTON (AP) — President Barack Obama's choice to run the Pentagon earned the support Thursday of the Senate's top Republican, who called on the nominee to "speak truth to power" to both Congress and the commander in chief.

Senate Majority Leader Mitch McConnell, R-Ky., said he would back the nomination of Ashton Carter, who would be Obama's fourth defense secretary in six years. The 60-year-old Carter, a physicist by training, is on track to secure Senate confirmation.

In offering his endorsement, McConnell said Obama administration policies have weakened the country's ability to battle al-Qaida and its affiliates, the Taliban and the Islamic State group. He said Obama has been inflexible about breaking his campaign promises to draw down forces in Iraq and Afghanistan and close the U.S. detention center in Guantanamo Bay, Cuba, that holds suspected terrorists.

"If I could place one demand on him," McConnell said of Carter, "it would be to leave his successor with our armed forces in a better position to deal with global threats than they are today."

McConnell said his support for Carter is conditioned on this question: "The incoming secretary needs to have the courage to speak truth to power — to Congress, yes, but also to his commander in chief."

While moving ahead on Carter, Senate Republicans delayed a committee vote on Loretta Lynch, Obama's nominee for attorney general. Lawmakers said they had more questions for Lynch, the U.S. attorney for the eastern district of New York who would become the nation's first black female attorney general if confirmed.

Democrats complained that Lynch is being held to a double standard compared to other nominees and said her nomination should be approved immediately. Her confirmation is expected after Congress' one-week recess.

Carter would replace Chuck Hagel, the Vietnam War veteran and former Republican senator from Nebraska who struggled with Obama's tight-knit group of national security advisers.

Carter served as the Pentagon's second-ranking official from 2011 to 2013, spent two years previously as the department's technology and weapons-buying chief and was assistant secretary of defense for international security policy during Bill Clinton's administration.

As defense secretary, he will face the unenviable task of steering the military as the United States tries to defeat Islamic State militants, stop Russian-backed separatists in Ukraine and wind down the war in Afghanistan. He also will face smaller Pentagon budgets.

His expected Senate confirmation comes as the administration is seeking new congressional authority for its military campaign against the Islamic extremists.

While he has the resume for the job, it's unclear whether Carter would find more success than Hagel in jelling with Obama's inner circle. The president's relationship with the Pentagon has often been strained.

Sen. John McCain, R-Ariz., chairman of the Armed Services Committee, alluded to that divide on Thursday in reiterating his support for Carter, saying he would cast his vote with "sincere hope and, sadly, little confidence that the president who nominated Dr. Carter will empower him to lead and contribute to the fullest extent of his abilities."

At his confirmation hearing last week, McCain told Carter he hoped he would push back on any attempt by the White House to micromanage the Defense Department, or over-centralize U.S. foreign and defense policies.

"I'll be entirely straight and upfront with the president and make my advice as cogent and useful to him in making his decisions as I can," Carter promised.

Carter told the committee he was inclined to support providing defensive lethal aid to Ukrainians battling Russian-backed separatists. He also said he would consider recommending a change of plans for withdrawing all U.S. troops from Afghanistan by the end of 2016 if security conditions worsen. About 10,600 U.S. troops remain in Afghanistan.

Carter said 2,106 American service members have lost their lives in Afghanistan. "Finishing the job there is very important," he said.