WASHINGTON—The two major presidential rivals sharpened their long-standing dispute over the Iraq War on Tuesday, Democratic Sen. Barack Obama calling it a costly distraction that must end while Republican Sen. John McCain insisted it is a conflict the United States has to win.

“Iraq is not going to be a perfect place, and we don’t have unlimited resources to try and make it one,” Obama said in a speech in which he also said the United States must shift its focus to defeating the Taliban and al-Qaida in Afghanistan.

Rebutting swiftly, McCain said Obama “will tell you we can’t win in Afghanistan without losing in Iraq. In fact, he has it exactly backwards.”

While the two men agreed on the importance of prevailing in Afghanistan, the dispute veered in a new direction when it came to the tribal areas of next-door Pakistan, where terrorist Osama bin Laden and his men are thought to be hiding.

McCain accused Obama of “trying to sound tough” by speaking publicly of taking unilateral action against those blamed for the terror attacks of Sept. 11, 2001.

Undeterred, Obama said, “If Pakistan cannot or will not act, we will take out high-level terrorist targets like bin Laden if we have them in our sights.”

Despite his rhetoric, Obama refrained from saying the administration’s so-called surge in troop strength in Iraq had failed. Aides said his campaign Web site had been altered in recent days to remove references to that effect.

The two men sparred as Obama looked ahead to an overseas trip that will include stops in Iraq and Afghanistan, two countries where thousands of U.S. combat forces are engaged in combat as part of the global war on terror. Given differences in age and experience between the two rivals, Obama’s trip has taken on elements of an audition for a man seeking overall charge of U.S. war policy as well as foreign policy in general.

A Washington Post poll released before the two men spoke showed the country evenly divided on whether Obama, 46, now serving his first term in the Senate, would be a good commander in chief. favorably on the same question.