"Important", a new three-minute ad directed by Whedon, features 27 celebrities, including Robert Downey Jr, Scarlett Johansson, Mark Ruffalo and Don Cheadle – who appear in The Avengers series – as well as Neil Patrick Harris, Julianne Moore and Stanley Tucci.
The ad does not name Trump or explicitly endorse his main opponent, Hillary Clinton – but the implication is that voters should put their support behind Clinton in order to prevent Trump reaching the White House.
"You only get this many famous people together if the issue is one that truly matters to all of us: a disease, or ecological crisis, or a racist, abusive coward who could permanently damage the fabric of our society," various members of the cast say.
Later in the ad, Hollywood veteran Martin Sheen says: "We cannot pretend both sides are equally unfavourable."
The spot also humorously plays with the trope of gathering together celebrities to influence public behaviour. Julianne Moore describes the cast as "just a shit ton of famous people", but the ad also alludes to the fact that some of the individuals appearing are rather less well-known – as well as poking fun at the perceived vanity of Hollywood actors.
And it concludes with a promise that if Clinton wins on 8 November, Mark Ruffalo, who plays the Hulk in the Avengers films, will do a nude scene in his next flick – apparently without the knowledge or consent of Ruffalo.
"Mark’s gonna have his dick out," promises Robert Downey Jr.
The ad is funded by Save The Day, a super PAC (political action committee) founded by Whedon to support the Clinton campaign. It has already donated $1m to the Democratic candidate’s campaign, the Hollywood Reporter writes. Save The Day will produce a series of 15-25 videos between now and election day.