As the election in the US nears, the message from the Vatican is clear - Catholics should make their moral choices and voices heard in the ballot box.

In recent weeks, Vatican officials from Pope Benedict down have been sprinkling their speeches with comments on the need for Catholics to pin their moral colours to the political mast.

"Religion is not like smoking," Pope Benedict's secretary of state, Cardinal Tarcisio Bertone, told a conference on religion and politics. "It is not something that can be tolerated in private but strictly controlled in public."

Archbishop Gianpaolo Crepaldi, number two at the Vatican Council for Justice and Peace, said this week Catholics should not be caught "dozing off" when political choices were made and warned that "God cannot be left in the pew".

Last week Archbishop Raymond Burke, a senior American in the Vatican, said the Democratic Party risked "transforming itself definitively into a party of death" because of its choices on bioethical questions and abortion.

Those words took off in the blogosphere, with some commentators attacking him for trying to influence the election and others applauding what they said was his courage.

Apart from Archbishop Burke's, most Vatican comments have been general, with no specific reference to the US election and no clear evidence of a conscious strategy.

But they have emerged with curious regularity over the past few weeks in a period when no other major elections are in the offing.

"American elections these days are by definition global events. They create a kind of ferment in which the question of the Church's role in politics is just inevitably on people's brains," said John Allen, American author and religious affairs commentator.

"What's going on in the States today is inevitably creating a situation in which Church leaders feel they have to say something, wherever they are," he said.

Archbishop Burke, the Vatican-based US archbishop, accused the Democratic Party's most high-profile Catholics vice presidential candidate Joe Biden and House Speaker Nancy Pelosi - of misrepresenting Church teaching on abortion.

"While presenting themselves as good Catholics, (both) have presented Church doctrine on abortion in a false and tendentious way," he said.

In the last two months US bishops have chided Mr Biden and Mr Pelosi for misstating Catholic teaching. Both politicians are pro-choice and have said abortion, which the Catholic Church says is murder, is a personal decision.

But abortion has taken a back seat to many other issues in the presidential campaign, such as the economy, security and immigration. It did not come up in Thursday night's debate between Mr Biden and Alaska Governor Sarah Palin.