Learn English – What does “binder” mean? Why did it become a political buzzword

I’ve been seeing a lot of “binders” in recent newspaper and magazine articles dealing with the recent Presidential debates. For examples:

Time magazines October 19 issues carries the article titled, “Romney’s Binders: The Meme Women Love to Hate – How one little phrase became a potent political symbol.

There was another article in the same issue titled, “Obama and Romney dish out jokes, Not Jabs, at Annual Al Smith Dinner,” followed by the paragraph:

"At the outset, host Al Smith IV, the director of the Alfred E. Smith
Memorial Foundation — first acknowledged women in the room and said,
“It’s good to see who’s getting out of those binders.” It was a quip
that proved the proliferation of what’s become a new meme, after
Tuesday’s debate when Mitt Romney explained that while serving as
governor of Massachusetts, he was provided with “whole binders full
of women” to help him fill his Cabinet.

In the article of New Yorker magazine October 19 issue titled “Obama on “The Daily Show”: A Gaffe is a Gaffe,” there comes again “Binder full of women”:

“When he (Romney) said, on Tuesday night, that as governor of
Massachusetts he got help from women’s groups in staffing his
administration – “they brought us whole binders full of women”-he
was trying to come across as enlightened on gender issues, but managed
to give the opposite impression.”

My favorite Maureen Dowd also quotes “women in binders” in her articles, “Pampered princes fling Gorilla dust” (October 20) and “Of Mad men, Mad women and Meat loaf” (October 27) on NY Times:

“Obama’s contempt for Romney gleamed through as Mitt got all O.C.D.
with Candy Crowley about the rules, and rambled on about his weird
retro worldview, where women in binders have to bound home to make
dinner, – – where we just tell “our kids that before they have babies,
they ought to think about getting married to someone.”

“Mitt hopes Americans are ready for some rules — and binders. He is
baked in the fuddy-duddy dad image from the era when white men ruled
and the little women toiled over a hot stove.”

Oxford Dictionaries defines ‘binder’ as:

1. a cover for holding magazines or loose sheets of paper together.

2. a substance used to make other substances or materials stick or mix together.

3. a reaping machine that binds grain into sheaves.

4. a bookbinder.

However, the word, ‘binder’ seems to be becoming a ‘new meme or symbol’ as the authors of both of the above articles say, containing different meaning. What does it mean in those contexts?