In an op-ed piece published in the New York Daily News, Paul said the slaying of a U.S. citizen without due process is a violation of the Constitution and sets a dangerous precedent.
"The precedent set by the killing of (Anwar) al-Awlaki establishes the frightening legal premise that any suspected enemy of the United States -- even if they are a citizen -- can be taken out on the president's say-so alone," said Paul, a Texas congressman seeking the Republican presidential nomination.
Paul, who enjoys support from libertarians and Tea Party activists, is considered a dark horse for the nomination. He warned that allowing President Obama to act as "judge, jury and executioner by presidential decree" would open the door to other politicians circumventing the law for selfish reasons.
"I don't trust Obama with determining what protections I should be allowed as a citizen any more than I trust him with our general defense, the economy, healthcare, job growth -- or anything else," Paul said.