When to use "GOP" versus "Republican Party"

I've read lots of professional articles that use the two terms interchangeably, sometimes within the same paragraph. Are the two terms really semantically identical? In formal writing, are there any situations where it would be recommended to use one or the other?

In more formal writing, it's slightly proper to use the phrase "Republican Party". However, in general discussion or news reporting, you'll find it used interchangeably.

GOP (Grand Old Party) is a nickname for the Republican Party that dates back over a century. The use of the word 'old' is ironic in that the Democratic Party was founded before the Republican Party. Also, some of the alternate meanings of the word 'grand' cause members of opposing parties to use the term GOP sarcastically, or not at all.

It is favored in news headlines due to its shortness.

Wikipedia mentions:

The term "Grand Old Party" is a traditional nickname for the Republican Party, and the initialism "G.O.P." (or "GOP") is a commonly used designation

, and refers to the OED:

The first reference to the Republican Party as the "grand old party" is dated to 1876; the first use of the abbreviation "GOP" is dated 1884.

Both terms are fairly interchangeable, and while GOP is mainly used in headlines, it is interesting to know that, as detailed in the article "What Does 'GOP' Stand For?", in 2002 The Wall Street Journal actually had decided to stop(!) using the acronym to refer to the 148-year-old political party.

In an internal memo issued to staffers last week, Journal higher-ups said the term GOP will be dropped because not all readers know what the letters mean, and some may not realize that they are a reference to the Republican Party.

Titles like "Democrats Aim to Curb GOP Donors" (on the online Wall Street Journal site) show that this resolution didn't stick.