But Obama had opposed the Iraq invasion and now worries that an open-ended U.S. combat mission here will sap military resources and focus — at a time when Iraq violence has dropped to its lowest level in four years.

The Illinois senator — traveling with Sens. Jack Reed, D-R.I., and Chuck Hagel, R-Neb. — arrived first in the southern city of Basra, the U.S. Embassy said.

Basra is the center for about 4,000 British troops involved mostly in training Iraqi forces. An Iraqi-led offensive begun in March reclaimed control of most of the city from Shiite militia believed linked to Iran.

His meetings in Baghdad were expected to include the top U.S. commander in Iraq, Gen. David Petraeus, and other military chiefs outlining the significant gains in recent months against both Shiite militia and Sunni insurgents including al-Qaida in Iraq.

The White House and military leaders — and many residents of Baghdad — trace the momentum back to last year's buildup of more than 30,000 troops in areas around Iraq's capital. McCain has tried to hammer Obama on his criticisms of that military surge.

In an interview Monday on ABC's "Good Morning America," McCain said he hoped Obama would now "have the opportunity to see the success of the surge."

"This is the same strategy that he voted against, railed against," McCain said. "He was wrong about the surge. It is succeeding and we are winning."

All five surge brigades have left Iraq, but there are still about 147,000 U.S. soldiers in Iraq, more than in early 2007.

Iraqi leaders also pressed Obama for more clarity on his long-term vision for relations with Washington. Such discussions have added importance since Iraq and U.S. negotiators appear stalled in efforts to reach a long-range pact to define future U.S. military presence and obligations.

American diplomats hoped to reach a final accord by the end of the month, but it now seems the goal is a stopgap "bridge" document that would maintain the status for U.S. forces once a U.N. mandate on their presence expires at the end of the year. Such as move would leave the hard bargaining to the next president.

Obama arrived following talks Sunday in Kuwait with the emir, Sheik Sabah Al Ahmed Al Sabah. Earlier he met with U.S. military commanders and troops in Afghanistan and held talks with President Hamid Karzai.

He is scheduled to go on to Jordan, Israel and European capitals.