It's the biggest day of the year for US advertising with companies spending between $2.5m and $2.8m to ensure their product is seen by the widest possible audience, but this year's Super Bowl Sunday threatens to be overshadowed by controversy over one of the 30-second slots.

The advert in question? A commercial on behalf of the evangelical Christian organisation Focus on the Family, featuring the University of Florida's star ­quarterback Tim Tebow and his mother Pam, which is expected to focus on her decision to ignore medical advice to have an abortion.

The almost $3m advert, which Focus on the Family says was paid for by donations, contravenes a network policy regarding the type of ads shown during the Super Bowl. Several online petitions have called on CBS to pull the ad and 2,288 people joined a Facebook group pointing out the hypocrisy by saying: "Tell CBS Reject The Focus On The Family Ad Or Accept The UCC's!" UCC refers to the United Church of Christ.

"Super Bowl ads are traditionally about making people laugh and, while there's no doubting the sincerity of Tebow's beliefs, I think people will find this jarring," says Clay Travis, an author and journalist who has written extensively about Tebow.

Rejected ads

"It's such a flashpoint subject and I'm surprised that CBS would go there after the fuss that was caused by Janet Jackson's nipple. It's not even a matter of whether you're pro-life or pro-choice, I think most people would find an advert dealing with abortion to be out of place during the Super Bowl," Travis adds.

Nor is it just a case of the ad appearing to be a bad fit with the Super Bowl's dancing lizards, singing frogs and magic fridges. Networks have previously made a point of rejecting advocacy adverts for Super Bowl slots – last year NBC rejected an anti-abortion advertisement on behalf of Catholicvote.org which used images of President Obama alongside the caption "Life. Imagine The Potential", in addition to one about marriage equality.

Nor were these groups alone – MoveOn.org and Peta are among those to have seen their commercials turned down while in 2004 CBS rejected an ad on behalf of the United Church of Christ targeting gay parishioners with the tagline: "Jesus Didn't Turn People Away. Neither Do We."

At the time, CBS claimed it had a policy of refusing advertising that "touches on and/or takes a position on one side of a current controversial issue of public importance". In contrast its response to the proposed Focus on Family commercial has been altogether more vague, stressing that "[CBS's] standards and practices continue to adhere to a policy that insures that all ads on all sides of an issue are appropriate for air". Officially the network has only approved the scripts and could still pull the advertisement before 7 February, but the general belief is that it is unlikely to do so.

So why this advertisement and why now? Focus on the Family takes a notoriously hardline stance on issues such as abortion and gay marriage. Its founder, James Dobson, has long been a polarising figure who infamously claimed that "homosexuals want to destroy the institution of marriage"; and the group's political wing, Focus on the Family Action, ran into trouble last year when it ran an ad on its website calling on members to pray for "a rain of biblical proportions" during Barack Obama's acceptance speech.

A year, however, is a long time in politics and while an anti-abortion advertisement would have been unlikely two weeks after Obama's inauguration, many social conservatives feel that with emotions running high over healthcare reforms and the economy, public feeling is swinging back towards them.

More important than that, though, is the man who will star in the commercial, the 22-year-old Tebow, who two years ago became the first sophomore player ever to win the prestigious Heisman Trophy, and is arguably more famous as a college quarterback than many NFL players, with eight Sports Illustrated covers and an ESPN documentary to his name.

On a mission

The home-schooled son of missionaries who wears black make-up referencing biblical passages during games, Tebow has repeatedly talked about his "mission", noting that football enables him to spread "God's word" and making it clear that his talent at the sport (he is expected to be one of the highest picks in this year's NFL draft) is secondary to his religious beliefs. "The key to understanding why they've agreed to this advertisement definitely lies with Tebow," says Travis. "Without him I don't think that CBS would have agreed the ad, he's the most famous religious figure under 40 in this country; but it's still an interesting decision to run the ad because once you've done so you can't go back. It opens the way for Super Bowl ads to become what they've never been – political."

And even within CBS not everyone is happy about that. In an impassioned piece which ran last week on the CBS sports website, columnist Gregg Doyel wrote about his unhappiness with the network, while admitting that the advert itself may make him cry. "I'm not complaining about the ad because it's anti-abortion and I'm not," Doyel wrote. "I'm complaining about the ad because it's pro-politics and I'm not. Not on Super Sunday … It's not a day to discuss abortion. For it, against it, I don't care what you are … [this] is simply not the day to have that discussion."

On 7 February CBS will find out whether the rest of America agrees.