ORLANDO, Fla. -- Sens. Barack Obama and John McCain escalated their debate over foreign policy Tuesday, as the Democrat struck back forcefully against charges that his views on the situation in Iraq are based on political calculation.

The Obama campaign also announced Tuesday that it will hold a rally Saturday at the Old State Capitol in Springfield, Ill., to begin the week of the Democratic National Convention. The candidate is expected to appear with his yet-to-be-named running mate at the gathering, party sources said. Obama announced his presidential candidacy at the same location on Feb. 10, 2007.

The presumptive Democratic nominee has a full plate in the days ahead. He will polish his acceptance speech while campaigning by bus through North Carolina and Virginia, two reliably Republican states that he hopes to move into the Democratic column. Party sources confirmed that former vice president Al Gore will speak on Thursday night at the convention, before Obama accepts the nomination at Denver's Invesco Field. The Obama campaign is also completing a highly secretive vice presidential selection process in which the front-runners are believed to be Sens. Joe Biden of Delaware, and Evan Bayh of Indiana and Virginia Gov. Timothy Kaine.

As he heads into a critical week, Obama has been engaged in an increasingly heated debate with McCain over the Iraq war and global terrorism. The candidates' back-to-back VFW speeches contrasted starkly different worldviews, with Iraq emerging as the center of their dispute.

McCain told the VFW crowd on Monday that Obama "cannot quite bring himself to admit his own failure in judgment," particularly about the 2007 troop "surge" that Obama vigorously opposed in the Senate.

"Senator McCain now argues that despite these costly strategic errors, his judgment has been vindicated due to the results of the surge," Obama responded Tuesday. Increasing U.S. troop levels did work, he conceded. "In Iraq, gains have been made in lowering the level of violence, thanks to the outstanding efforts of our military, the increasing capability of Iraq's security forces, the cease-fire of Shiite militias and the decision taken by Sunni tribes to take the fight to al-Qaida," Obama told the veterans. "Those are the facts, and all Americans welcome them."

But he added: "Understand what the essential argument was about. Before the surge, I argued that the long-term solution in Iraq is political -- the Iraqi government must reconcile its differences and take responsibility for its future. That holds true today."

Obama prefaced his critique with a nod to McCain's war record, including the years that the presumptive GOP nominee spent in a North Vietnamese prison camp. "He is a man who has served this nation honorably, and he correctly stated that one of the chief criteria for the American people in this election is going to be who can exercise the best judgment as commander in chief," Obama told the VFW.

"But instead of just offering policy answers, he turned to a typical laundry list of political attacks. He said that I have changed my position on Iraq when I have not. He said that I am for a path of 'retreat and failure.' And he declared that 'behind all of these claims and positions by Senator Obama lies the ambition to be president' -- suggesting, as he has many times before, that I put personal ambition before my country."

Instead, Obama asserted that McCain was twisting his words for political purposes. "If we think that we can use the same partisan playbook where we just challenge our opponent's patriotism to win an election, then the American people will lose," he said. "The times are too serious for this kind of politics. The calamity left behind by the last eight years is too great."

Obama pointedly questioned McCain's judgment on Pakistan and his loyalty to Pervez Musharraf, who resigned as president this week, which the Democrat suggested may have been costly in the fight against terrorism. "For all of his talk about following Osama bin Laden to the gates of hell, Senator McCain refused to join my call to take out bin Laden across the Afghan border," Obama told the crowd. "Instead, he spent years backing a dictator in Pakistan who failed to serve the interests of his own people."

McCain spokesman Tucker Bounds hit back. "Unlike Barack Obama, John McCain doesn't have to compensate for a lack of credibility on the international stage with inflammatory and public threats against American allies," he said in a statement. "The American people know that John McCain will hunt down terrorists wherever they are, and have a choice between strength and experience versus Barack Obama's rhetoric and theatrics."