"This election is far from over, and we're not inclined to make a final call until the first debate on Oct. 3, but this seems clear: the election is slipping away from Romney," said Greg Valliere of Potomac Research Group which analyzes Washington for investors.
But in a reminder of how campaigns can shift easily, Republicans began building a strong argument against Obama for failing to stop the rise of Islamists in Egypt and Libya, where the US ambassador was killed in an attack this week.
It is unclear whether Obama's lead in the polls is a residue of positive feeling from the convention, or due to more specific reasons like voter concern at the Republican ticket's plans for Medicare or Democratic ads attacking Romney's business record.
Poll leads at this stage in the race can evaporate quickly, as Republican Senator John McCain's did in the 2008 campaign.
But Obama seems to be doing something right to be able to stay ahead despite high unemployment, underscored by poor jobless figures last week. The Reuters/Ipsos poll showed Obama with a slight lead of 38 percent over 35 percent on which candidate has a better plan on jobs.
In key states, the numbers looked grim for Romney this week too.
An EPIC-MRA poll showed Obama up 10 points in Michigan, Romney's birthplace and a state where conservative groups supporting him pulled back advertising dollars this week.
A poll released on Tuesday showed Obama up 4 points in Ohio, one of the top three battleground states.