As the US Presidential campaign kicks into high gear, atheists across America are campaigning against their leaders' religious beliefs.

The American Atheists, a non-profit organisation has shelled out $15,000 has put up two huge billboards attacking Christianity and Mormonism, the religions of the two competing candidates, Barack Obama and Mitt Romney.

The billboards will be erected in Tampa, the site of the upcoming US Democratic Party convention.

The American Atheists also attempted to pay for the billboards in Florida – the site of the conservative Republican Party convention – but were turned down by local companies who called them 'attack ads'.

The billboard attacking Mr Romney's religion, Mormonism, claims it is the religion of 'big money and big bigotry'.

It also raises some of the more unusual aspects of Mormonism, saying the religion's god 'is a space alien' and it 'baptizes dead people' and mentions magic underwear.

Mr Romney's religion has been the focus of significant attention in the lead up to the election.

Critics have noted Mormonism has a history of supporting polygamy, racism, sexism and slavery.

Mr Romney, the first ever Mormon nominee for the US Presidency has addressed the issue, saying he's proud of his Mormon roots and how far the religion has come on difficult issues in recent decades.

The billboard attacking President Obama’s religion, Christianity, says it 'Promotes Hate, Calls it "Love."

The American Atheists say they hope the billboards will highlight the dominance of organised religion in US politics, and will provoke voters to think about their choices.

"The election of our leaders in the United States is one of the most important decisions that we as citizens make. Allowing our judgment to be clouded by sheer silliness is unacceptable," David Silverman, president of the American Atheists, said in a statement.

"We want to show the people of our country the foolishness of mixing religion with politics."

The separation of church and state, which is written into the US constitution, is still a hot-button issue in the US, where religious beliefs dominate the Congress.

Recently a Missouri politician sparked outrage by saying women who are the victims of a 'legitimate rape', have ways of preventing themselves from getting pregnant.

The Republican Party's presumptive nominee for Vice President, Paul Ryan, supports a 'personhood amendment', which would give a foetus the same rights as any human.

Highlighting the significant role religion plays in US politics, Mr Ryan's position clashes with Mr Romney, who believes in legal abortion in cases of rape or incest.

Mr Ryan has called Mr Romney's abortion position 'a step in the right direction'.

Previously, no President of the United States has been an atheist.

In 1960, concerns over John F. Kennedy's Catholicism grew so strong the candidate was forced to address the issue in a significant speech.