Republican John McCain again insisted victory in Iraq “is finally in sight.” So why, asked Democrat Barack Obama, does his presidential opponent still oppose a timetable for withdrawing American forces.

The argument over the unpopular war — a key issue separating the candidates — flared again Monday after McCain spoke to the Veterans of Foreign Wars convention, a friendly audience given his service as a Navy fighter pilot and prisoner of war in North Vietnam.

Overshadowing the back and forth this week is the mounting speculation about who Obama will choose to be his vice presidential running mate. An announcement is expected within days. Obama is believed to have narrowed the field to Virginia Gov. Tim Kaine, Delaware Sen. Joe Biden, Indiana Sen. Evan Bayh and Kansas Gov. Kathleen Sebelius. His vanquished rival, New York Sen. Hillary Rodham Clinton, also remained a possibility.

The Iraq war, the top election issue before the presidential contest began, has given way to voter concerns over a stumbling economy that shows no signs of rebounding as Democrats head into their national convention next week. Republicans gather to nominate McCain the following week.

McCain, a four-term Arizona senator, said the victory he envisioned still could be “squandered by hasty withdrawal and arbitrary timelines. And this is one of many problems in the shifting positions of my opponent, Sen. Obama.”

Obama issued a sharp retort, questioning McCain’s sincerity in support of Iraqi sovereignty — one of the Bush administration’s goals in the war McCain heavily promoted in the months after the Sept. 11 terror attacks.

“It is hard to understand how Sen. McCain can at once proclaim his support for the sovereign government of Iraq, and then stubbornly defy their expressed support for a timeline to remove our combat brigades from their country,” said Obama campaign spokesman Bill Burton.

“John McCain is intent on spending $10 billion a month on an open-ended war, while Barack Obama thinks we should bring this war to a responsible end and invest in our pressing needs here at home.”

McCain insists U.S. troop withdrawals be dependent on conditions on the ground in Iraq. Obama, with agreement of the Baghdad government, says American forces should be gone in 2010.

Obama, who has been put on the defensive by a series of attacks on his character, experience and readiness for the presidency, has been responding to the McCain accusations blow for blow.

The Illinois senator has sought to defang McCain’s attempt to make the 2008 presidential contest a referendum on Obama while trying to duck his associations with the unpopular President Bush.

In a bid to counter McCain claims about Obama’s thin resume and short national political profile, the freshman Illinois senator was widely expected to reach for a fellow Democrat with solid foreign affairs and security credentials when he finally names a vice presidential running mate this week.

Among the possible choices is Biden, chairman of the Foreign Relations Committee who undertook a weekend trip to Russian-occupied Georgia. Biden, who challenged Obama in the Democratic primaries, has been in the Senate since 1972 and is one of its foreign policy deans.

Other top contenders for the No. 2 spot on Obama’s ticket, both with far less heft than Biden, are Bayh, a middle-of-the-road Indiana senator with an extensive Democratic pedigree, and Kaine, who leads a Republican-leaning state Obama needs to put in his column on Nov. 4.

McCain, also, needs to name a running mate.