U.S. Republican presidential nominee Donald Trump wrote a letter to the country's pro-life leaders and vowed to defund abortion service provider Planned Parenthood once he wins the White House.

"As we head into the final stretch of the campaign, the help of leaders like you is essential to ensure that pro-life voters know where I stand, and also know where my opponent, Hillary Clinton, stands," he writes.

Trump began by saying that Clinton not only supports abortion on-demand but also plans to take it a step further by forcing taxpayers to fund abortions by repealing the bipartisan Hyde Amendment.

"Hillary Clinton also supports abortion until an hour before birth. And she will only appoint Supreme Court justices who share this view. She doesn't even try to hide her extremism. When asked on 'Meet the Press' when unborn children have constitutional rights, Clinton bluntly responded, 'The unborn person doesn't have constitutional rights,'" Trump says.

The Democratic nominee is so committed to that view that she even said that "religious beliefs...have to be changed" to make way for her abortion agenda, Trump says.

In contrast, the real estate magnate says he plans to "advance the rights of unborn children and their mothers" when he is elected president.

He says he is committed to nominating pro-life justices to the U.S. Supreme Court and signing into law the Pain-Capable Unborn Child Protection Act, which would end painful late-term abortions nationwide. The business magnate also plans to reallocate the Planned Parenthood fund to community health centres that would provide comprehensive health care for women.

"Your help is crucial to make this contrast clear in the minds of pro-life voters, especially those in battleground states. Together we can form this vital coalition so that Mike Pence and I can be advocates for the unborn and their mothers every day we are in the White House," says Trump.