Senator Rand Paul (R-KY) is a viable presidential candidate as he is the only major party candidate who's positions on foreign policy are supported by a majority (47%) of Americans (1) and can beat Hillary Clinton using his/her record on civil liberties and foreign policy (her vote for the Iraq War).
Senator Paul can't be a viable candidate as he couldn't even make it through a GOP primary as his views on foreign policy and social issues are untenable to most GOP voters. For example, he believes that the Civil Rights Act of 1964 is unconstitutional and that the minimum wage shouldn't exist at all and even most Republicans don't believe that.
"his views on foreign policy [...] are untenable to most GOP voters" 53% of Republicans support his position of non-interventionism (1). Paul is pro-life like the vast majority of Republican voters. "he believes that the Civil Rights Act of 1964 is unconstitutional" Paul believes in a businessman's right to associate with ever they want to (If you were Jewish, would you want to be forced to sell [whatever] to a Nazi). Also, unlike Christie or Bush, Paul starts out with a dedicated base of supporters and campaign infrastructure "leftover" from his father's campaign.
Only hard-core racists oppose the Civil Rights Act of 1964 and his father will be a liability because he is one (1). Non-interventionism is very dangerous as it allows terrorists to expand and win around the world. But it is even more dangerous that a candidate like Rand Paul supports non-interventionism and is being presented as a leading presidential contender. Hillary Clinton for President!