Hollywood may have run out of summer hits, but an anti-Obama documentary is helping to fill the gap.

Holdover movies easily topped the weekend box office again, led by Sylvester Stallone's The Expendables 2 at No.1 for the second-straight weekend with $US13.5 million ($A13 million).

The weekend's new wide releases were overshadowed by 2016: Obama's America, which expanded from limited to nationwide release and took in $US6.2 million to finish at No.8.

The documentary is a conservative critique envisioning what the country will look like four years from now if President Barack Obama is re-elected.

Released by Rocky Mountain Pictures, Obama's America nearly matched the $US6.3 million debut of the No.7 movie, Joseph Gordon-Levitt's action tale Premium Rush, a Sony release that played in more than twice as many theatres as the Obama documentary.

The weekend's other new wide releases opened weakly. Dax Shepard and Kristen Bell's road-chase comedy Hit Run, released by Open Road Films, debuted at No.10 with $US4.7 million, and the Warner Bros fright flick The Apparition opened at No.12 with $US3 million.

The weak openings are typical of late August, a dumping ground for movies without much audience appeal as the summer blockbuster season winds down and young viewers switch to back-to-school mode.

But with less competition from Hollywood releases, it also opens the door for surprise successes such as Obama's America.

'It's extremely rare for a documentary to break into the top-10, but August can be a land of opportunity for smaller films,' said Paul Dergarabedian, box-office analyst for Hollywood.com. 'Also, there's the fact that this is a very conservative film. Normally, it's Michael Moore-branded documentaries, the liberal documentaries that make all the money.'

Obama's America opened in a handful of theatres in mid-July and did strong business as it gradually widened to more cities. It jumped into the top-10 this weekend as it expanded into 1091 theatres, leading all other wide releases with an average of $US5717 a cinema.

That's a solid average, especially for a political documentary. But it pales next to the king of political documentaries, Moore's George W Bush assault Fahrenheit 9/11, which opened at No.1 with $US23.9 million in June 2004, averaging $US27,558 in 868 theatres. Fahrenheit 9/11 went on to become the top-grossing documentary ever with $US119.1 million domestically.

Obama's America is based on the book The Roots of Obama's Rage, written by Dinesh D'Souza, who co-directed the movie with John Sullivan.

The documentary now has climbed to a $9.1 million domestic total, with prospects for strong business as the Republican National Convention unfolds over the next few days.

D'Souza said Obama's America will expand to more cities in the coming weeks and probably remain in theatres until early October, likely followed by a DVD release in the home stretch leading up to the November 6 election.