Republicans keen to disrupt Barack Obama's run-in to his big convention speech pounced Tuesday after the president awarded himself only a grade of "incomplete" on fixing the ravaged economy.

The US leader was in politically contested territory in Virginia to hold his final rally before heading to the Democratic National Convention, which his wife Michelle was to headline with an opening night speech later in the day.

Republicans are trying to narrow the election into a referendum on Obama's economic management, and capitalized on several days of clumsy messaging by the president and his team, inserting themselves into convention news coverage.

Obama was asked to grade his performance on the economy during an interview with a Colorado news program broadcast on Monday and unwittingly provided an opening for his Republican foe Mitt Romney and his team.

"You know, I would say 'incomplete,'" Obama said, before laying out steps he had taken to save the auto industry, make college more affordable and to invest in clean energy and research, which he said were important for the long term.

"We are still going through one of the toughest times that we've had in my lifetime and because of the financial crisis we lost nine million jobs," Obama told KKTV.

Republican vice presidential nominee Paul Ryan quickly took Obama to task over the remark, which illustrated how any offhand comment can suddenly be fodder for daily campaign combat in a close election.

"Four years into a presidency and it's incomplete?" Ryan told CBS News Tuesday.

"The president is asking people just to be patient with him. Look ... the kind of recession we had, we should be bouncing out of it, creating jobs.

"We're not creating jobs at near the pace we could."

The Republican assault came on the heels of another stumble by the Obama team at the weekend, when a list of top campaign officials struggled to answer the question: "Are Americans better off now than they were four years ago?"

Ryan was again the antagonist when he compared Obama on Monday to former Democratic president Jimmy Carter, who was felled by Ronald Reagan in 1980, partly owing to an anemic economy.

"The president can say a lot of things, and he will, but he can't tell you that you're better off," Ryan said, mounting a raid into North Carolina on the eve of the Democratic nominating convention in Charlotte.

The Obama campaign sent out Joe Biden to answer the charge, in characteristically bombastic fashion.

"Folks, let me make something clear, say it to the press -- America is better off today than they left us," Biden said, referring to the situation left behind by the previous Republican administration.

"Let me just sum it up this way folks ... Osama Bin Laden is dead and General Motors is alive."

The question of "are you better off after four years" is a perennial feature of US presidential campaigns, and poses a puzzle for the Obama campaign.

If Obama aides answer that Americans are better off, they could be seen as indifferent to economic struggles of middle class voters.

But if they say America is not better off, they immediately undermine Obama's rationale for seeking a second term in office in November's election.

Republicans deepen their dilemma by brandishing figures showing high youth unemployment and by noting the 8.3 percent national jobless rate and weak economic growth rates.

Polls show that while Obama is seen as more likeable than Romney, more trusted to protect the middle class and favored on foreign policy, he pays a price for the turgid economy, a factor that is keeping the race close.

A new opinion survey of likely voters by The Hill newspaper released Tuesday reflected the delicacy of the president's position.

Fifty-two percent said that America was in a worse condition than in September 2008, when the financial crisis struck, leaving Obama vulnerable to complaints his economic plans failed to spark a sufficiently quick turnaround.

Some 54 percent of those polled said that Obama did not deserve a second term based on his job performance.

Yet despite those figures, and partly due to Romney's own problems wooing the electorate, the race remains deadlocked, mostly within the margin of error in national polls and in surveys of key battleground states.

Obama's Virginia stop followed a four-day tour of key electoral territory in Iowa, Colorado and Ohio. He will fly into Charlotte on Wednesday, on the eve of his crucial prime time address.