Senator Barack Obama woke up on Wednesday talking of his delegate lead and of taking the fight to Senator Hillary Rodham Clinton. But after defeats in two of the most populous states, he also sounded like a chastened candidate in search of his lost moment.

Mr Obama once again failed to administer an electoral coup de grâce, and so allowed a tenacious rival to elude his grasp.

Now, after appearing nearly invincible just last week, he faces questions about his toughness and vulnerabilities — never mind seven weeks of tramping across Pennsylvania, the site of the next big primary showdown. His goal is to prove he can win states vital to a Democratic victory in November.

Flying from Texas back home to Chicago on Wednesday morning, Mr Obama delivered the message that he intended to counterpunch forcefully.

His campaign aides on Wednesday urged Ms Clinton to release her tax returns from 2006, as well as her papers from her years as first lady, which Mr Obama’s chief strategist, David Axelrod, described as “secreted in the Clinton library.”

Mr Obama took aim on Wednesday at Ms Clinton’s claim that she is a seasoned hand in foreign policy. “What exactly is this foreign experience that she’s claiming?” he said. “I know she talks about visiting 80 countries. It is not clear. Was she negotiating treaties or agreements or was she handling crises during this period of time?” Mr Obama, finally, has tactical worries of his own. He won in states like Missouri by running up large margins in cities and suburbs. But in Ohio, he appeared outorganised.

He retains significant advantages, including his lead among pledged delegates and a record-setting fund-raising operation.

And he bridled at questions about his difficulties attracting working-class and middle-class support, noting his progress in that regard. “In Missouri, Wisconsin, Virginia and many of these states, we’ve won the white vote and the blue-collar voters. I think it is very important not to somehow focus on a handful of states because the Clintons say that those states are important and the other states are unimportant.”