Question: Is it proper grammar to write July 17th or would it be the 17th of July?

Answer: 
the **late-night** part comes from:

[Late-night talk show](http://en.wikipedia.org/wiki/Late-night_talk_show)

> 
> A late-night talk show is a subgenre of the talk show genre. In
>  American television at least, it is specifically a kind of
>  comedy-oriented talk and variety show that airs late at night.
>  Characteristics of the genre include topical monologues in which the
>  host makes fun of the day's news, comedy sketches, celebrity
>  interviews, and musical performances.
> 
> 
> 

and **punchline**

> 
> The climactic phrase or statement of a joke, producing a sudden
>  humorous effect.
> 
> 
> 

Basically late-night punch line just means the butt of a joke on a talk show.

note: late-night jokes are not the classiest.

**edit**:

The sentence "*went from potential presidential contender to late-night punch line*" means that the South Carolina governor originally was very highly esteemed, even to the point that people thought he was going to be the next president but, and I don't know what happened, something happened and he turned into a joke. No it does not mean that he went from an unknown to becoming the popular object of late night TV - but that on his way 'up the ladder' he crashed and burned.
The proper parsing is: "The governor went from X to Y." X equals "presidential contender." Y equals "late night punch line."

So the phrase means, "The governor used to be a Presidential contender. Now he is a late night punch line (aka, a "joke.")

There is no such phrase as "contender to late night punch line." That is to say, "contender" is connected to "Presidential," not to "late night punch line."

The reason was revelations about his personal life, specifically his dating a woman not his wife, in a state where marital fidelity is particularly highly valued.

From the linked article, "Before his love affair became international news," 
he was a potential candidate for President of the United States (presidential contender). Afterward, he was a "joke," or verbal "punching bag" on late night shows.