In a statement, Mrs. Clinton described General Petraeus as “an able and respected leader in Iraq under incredibly difficult circumstances,” and said she looked forward to hearing “how he will meet these important challenges” of the broader Central Command region.

Mr. McCain, at a news conference on Wednesday, said General Odierno “is maybe not perfect, but I think he has done a magnificent job.” Referring to General Petraeus, Mr. McCain said, “I think he is by far the best-qualified individual to take that job” as the regional commander.

After three tours in Iraq, General Petraeus, 55, has become perhaps the best-known military officer of his generation, and it had been expected that his next assignment after Iraq would be as the top American commander in Europe. Chosen instead to take charge of a region that includes Pakistan and Iran, as well as Iraq and Afghanistan, General Petraeus issued a statement on Wednesday saying, “I am honored to be nominated for this position and to have an opportunity to continue to serve.”

General Petraeus and General Odierno have built a strong working relationship and are believed to see eye to eye on how to carry out the complicated Iraq mission  one they believe requires offensive military operations, more subtle counterinsurgency missions and society-wide reconstruction, all at once.

Mr. Gates said General Odierno was the logical choice to succeed his old boss because he was familiar to the officers and troops in Iraq and, not least, to the Iraqis. “In most parts of the world, especially the Middle East, personal relationships make a big difference,” Mr. Gates said.

The defense secretary also announced that Lt. Gen. Peter W. Chiarelli, Mr. Gates’s senior military assistant, would be nominated for Army vice chief of staff, a post that General Odierno had been expected to take. General Chiarelli has had two tours in Iraq  first as commander of the First Cavalry Division and coalition forces in Baghdad, and then as the No. 2 commander in the country.

The Central Command position would be General Petraeus’s fourth tour in the region since the invasion of Iraq in 2003. He first served as commander of the 101st Airborne Division, which invaded Iraq from the south and set up an area of control across the north. But parts of the north, in particular the city of Mosul, are now among the most unstable in the nation.