Mitt Romney's trying hard to show he's a bona fide conservative, and Friday spoke to the National Rifle Association national convention in St. Louis. And he invoked a Democratic president--Harry S Truman--to make his point.

Here's an excerpt from his address, taken from his prepared remarks:

"It was one of Missouri’s greatest sons, Harry Truman, who expressed a guiding conviction that you and I share. In a ceremony that placed the Constitution and Declaration (of independence) in the permanent care of the National Archives, President Truman offered a word of caution.

"Liberty, he said, “can be lost, and it will be, if the time ever comes when these documents are regarded not as the supreme expression of our profound belief, but merely as curiosities in glass cases.”

Romney, the presumptive GOP presidential nominee this year, went on to explain "Truman believed, as we do, that the principles of our Constitution are enduring and universal...that they were not designed to bend to the will of presidents and justices who come and go.

"The belief that we are all created equal, that we are endowed by our Creator with unalienable rights – these are not relics from another time, they reflect truths that are valid in every era. The framework of law created by the Declaration and the Constitution is the source of our greatness.

"It has generated unparalleled opportunity and prosperity. Our Founders understood this, which is why they created a system of government that is limited"

Then Romney went after President Barack Obama, saying, "this President is moving us away from our Founders’ vision. Instead of limited government, he is leading us toward limited freedom and limited opportunity. This November, we face a defining decision. I am offering a real choice and a new beginning.

"I am running for President because I have the experience and the vision to lead us in a different direction. We know what Barack Obama’s vision of America is – we’ve all lived it the last three years. Mine is very different. My course restores and protects our freedoms.

"A President, the Constitution would be my guide, and the Declaration of Independence my compass."