Pew Research Center for the People & the Press:

Obama continues to hold a sizable lead over Romney in the election contest. Currently, 51 percent say they support Obama or lean toward him, while 41 percent support or lean toward Romney. This is largely unchanged from earlier in July and consistent with polling over the course of this year. Across eight Pew Research Center surveys since January, Obama has led Romney by between four and 12 percentage points.

And:

By a 52 percent to 37 percent margin, more voters say they have an unfavorable than favorable view of Mitt Romney. The poll, conducted prior to Romney’s recent overseas trip, represents the sixth consecutive survey over the past nine months in which his image has been in negative territory. While Romney’s personal favorability improved substantially between March and June – as Republican voters rallied behind him after the primary season ended– his image has again slipped over the past month.

Barack Obama’s image remains, by comparison, more positive – 50 percent offer a favorable assessment of the president, 45 percent an unfavorable one. Even so, Obama’s personal ratings are lower than most presidential candidates in recent elections.

A review of final pre-election surveys of voters since 1988 finds that all candidates enjoyed considerably higher personal ratings going into the final days of their campaigns than does Mitt Romney currently. In fact, only three, Michael Dukakis in 1988, George H.W. Bush in 1992 and Bob Dole in 1996, were not rated favorably by a majority of voters. Obama’s current ratings also are lower than the pre-election ratings of most other recent presidential candidates.