Question

which president's views on the power of the federal government changed when he saw an opportunilty to authorize the louisiana

Answer 1

The Louisiana Purchase occurred in 1803. At this time, the president of the United States was Thomas Jefferson, who approved the purchase despite domestic disapproval.
So in short, your answer would be president Thomas Jefferson.
I hope this answer has helped, and I apologize if this answer somehow finds itself to be incorrect.