Rapper-turned-politician Kanye West has promised Americans to "revive our nation's commitment to faith" in the first campaign advert to aid his long-shot US presidential bid.

The 43-year-old, who's also a fashion designer, announced in July he was running for the White House.

But his campaign has been hobbled by a failure to secure his name on the ballot in multiple states. He has been asking voters to back him as a write-in candidate.

Now, just weeks before the November 3 election, the devoutly religious West has tweeted his first campaign video, which promises a focus on faith should the rapper succeed.

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"We as a people will revive our nation's commitment to faith, to what our Constitution calls the free exercise of religion, including, of course, prayer," West said in the advert, which featured a black-and-white American flag.

"Through prayer, faith can be restored. We as a people are called to a greater purpose than ourselves. We are not only a beacon to the world, but we should be servants to each other, to encourage each other, to help each other, to lift up each other, our fellow Americans, that we may all prosper together."

West, who has been vocal in his anti-abortion stance, promised to "build a stronger country by building stronger families" if he wins power.

West added: "Families are the building blocks of society, of a nation. By turning to faith we will be the kind of nation, the kind of people, God intends us to be."

Kanye West with Donald Trump at the White House

A message at the end of the video asks voters to "write in Kanye West". A write-in candidate is someone whose name does not appear on the ballot but is instead physically written in by a voter.

West used to be a supporter of Donald Trump, once describing him as a "brother".

He made a highly publicised trip to the Oval Office in October 2018 and discussed criminal justice reform with the president.

Bookmakers Ladbrokes have West at odds of 50-1 to win more than 5% of the popular vote.