A friend and I are debating this on Facebook, and I was curious to hear Dopers’ opinions. Here is the relevant FB excerpt, with identities redacted:
Well, in general, I’d define it to be ‘that block of voters considered most certain to vote for the party.’ Notice that they don’t need to voice support for the candidate/office-holder, or that they can’t voice criticism for him, as long as they’re not disaffected enough to make them stop voting for the party.
So generally, in a two-party system, the base would tend to skew towards the right-wing or left-wing, because those people don’t have alternatives, as opposed to the moderate centrists, who might switch their votes if offered a compelling candidate or platform that hits their hot button issues.
When third parties running extremist platforms are in the mix, it gets a little hard to pin down the bases. (And yes, even though bases are supposed to be stable, they can shift. It’s politics after all!)
I actually like D----- W-----'s definition of the base as those people who do the most work to get the party’s candidates elected. If you look at the Tea Party-type Republicans as a whole group – not just the elected ones, but the rank and file – a lot of them will split on individual issues, even high profile ones. But at the end of the day, the substantial majority of them vote for the party’s nominee, no matter how much a thorn in their side they may be.
So, someone like Jim DeMint, who has started a PAC to make mainstream Republican conservatives lose in primary contests to more Tea Party-oriented candidates. In the end, I’m sure DeMint voted for Mitt Romney. In my view, even though he does a lot to challenge Republican powers that be, as long as falls in line and helps to generate votes to win general elections for Republicans, I count him as part of the Republican party base… but not necessarily part of the Republican Party orthodoxy.
But what happens to that logic if these “base” voters support candidates in their parties’ primaries that are too far from the centre to win the general election? Doesn’t that mean they are not actually working to get their party’s candidates elected?

Well, in general, I’d define it to be ‘that block of voters considered most certain to vote for the party.’
…if they vote. They’re not going to defect, but they may be unmotivated.

But what happens to that logic if these “base” voters support candidates in their parties’ primaries that are too far from the centre to win the general election? Doesn’t that mean they are not actually working to get their party’s candidates elected?
I think that calls into question their judgment, but not their political orientation and activism.
But doesn’t a group of voters need to be helpful, essential in fact, to a party to qualify as the base?

But doesn’t a group of voters need to be helpful, essential in fact, to a party to qualify as the base?
The base doesn’t need to be totally lock-step in their support of the establishment and the conventional wisdom.
The establishment candidate in the 2008 Democratic primaries was Hillary Clinton. There were significant worries expressed that nominating Obama would be a mistake because he was inexperienced. It isn’t fair to exclude those who supported Obama in the primaries as not being “the Democratic base” because they didn’t go with the establishment candidate, and then start calling the same people “the Democratic base” once again after Obama won the general election. That’s just silly.
Well, but Obama had establishment figures on his side too: Tom Daschle, Harry Reid, Claire McCaskill. I’m talking more about when a party throws away an easily winnable election because their most fervid primary voters go too far to one side or another. In recent times it has mostly been in the GOP: they threw away gimmes in Indiana and Missouri this cycle, and last time in Nevada and Delaware.
The base are those that not only will vote for you, but that like/identify with you so much they will give you money, call their friends and tell them to vote for you, put out signs, etc.
If you “lose your base”, the fear isn’t so much that they won’t vote for you (though some may stay home), but that they won’t give you money, campaign for you, etc.
SmartAlecCat, this is how I see the base as well. But I think the key question here is: is it metaphysically possible for a party to be constantly losing, or on the verge of losing, its base? That is: if there is a group of diehard ideologues that constantly grouse about how watered down the party’s positions are, or how the majority of elected officials from that party are “INOs” (RINOs or DINOs, as the case may be), are they really the base? I would submit that the base has to be proud of, and enthusiastic about, their party and the majority of its elected officials most of the time. They can get restive on occasion if something or someone comes along and upsets the apple cart; but if they are constantly bitter and scornful, they just don’t qualify to be considered “the base” as I see it.
But the defections do happen. It happened in 2000. Not to all the Democratic base, but a sizable number of people who had been reliable Democratic voters voter for Nader instead because they thought Bill Clinton was a Republican in donkey’s clothes.
Now, the specific question you’re asking is how often, precisely, does a voter have to support his/her party to be considered the base? That is an unanswerable question to the same extent that if you have a bucket full of rocks, how many pebbles can you remove before the bucket is no longer full?
Sure, ultimately this is a semantic dispute. But that doesn’t mean one can’t have an opinion and advocate for it! My vehement opinion is that anyone who voted for Nader is excommunicated from being considered part of the base as I define it, unless they later saw the light and were extremely repentant.
I guess that’s the fundamental definitional question: whether “the base” of a party refers to the group of people who are ideologically furthest away from the *other *party in a two party system, but therefore regularly get frustrated with their own party and often support it only as a “lesser of two evils” proposition; or whether–as I strongly believe–it refers to a group that, more than anything, is enthusiastic and proud to be identified with their own party’s “brand”. By this definition, I am a signal member of the Democratic Party’s base, as are most African Americans from what I can tell. My fellow white progressives/liberals, though, quite often do not qualify as “base” by my definition. In fact, as much as I get frustrated with them for making the perfect the enemy of the good, they sneer at me for being a cheerleader for what they see as tepid, timid policies (or worse).

Sure, ultimately this is a semantic dispute. But that doesn’t mean one can’t have an opinion and advocate for it! My vehement opinion is that anyone who voted for Nader is excommunicated from being considered part of the base as I define it, unless they later saw the light and were extremely repentant.
I guess that’s the fundamental definitional question: whether “the base” of a party refers to the group of people who are ideologically furthest away from the *other *party in a two party system, but therefore regularly get frustrated with their own party and often support it only as a “lesser of two evils” proposition; or whether–as I strongly believe–it refers to a group that, more than anything, is enthusiastic and proud to be identified with their own party’s “brand”. By this definition, I am a signal member of the Democratic Party’s base, as are most African Americans from what I can tell. My fellow white progressives/liberals, though, quite often do not qualify as “base” by my definition. In fact, as much as I get frustrated with them for making the perfect the enemy of the good, they sneer at me for being a cheerleader for what they see as tepid, timid policies (or worse).
So dos the democratic base believe in supporting gay marriage?
I would say the Democratic base has just recently come around to supporting gay marriage. When Obama ran in the 2008 primaries he supported civil unions but not full marriage. His recent evolution to support of full marriage reflects the change of thinking among the base, I believe.