To what extent does an HRC presidential run pre-winnow the field before the Iowa Caucus?(Slightly edited for clarity)
That's a great question.
Democratic fields for open nominations have typically been fairly large, with one big exception: 2000, when only Bill Bradley challenged Al Gore. 2008 was typical, with six plausible nominees running (Obama, Clinton, Edwards, Dodd, Richardson, Biden). Similarly, in 2004, there were about seven plausible nominees who made it to Iowa. Democrats typically have done little winnowing between the "all-in" stage and Iowa; that is, there are candidates such as Mark Warner and Evan Bayh who say they are exploring runs and then get out, but few if any similar to Pawlenty, Liddy Dole, and Quayle, who mount full-on campaigns but then fold well before Iowa.
So 2000 really was unusual for the Democrats. Could 2016 be similar?
I'm afraid all I have here is a very strong and definitive...maybe.
What we need to ask, basically, are two things: is Hillary Clinton '16 similar to Al Gore '00? And: if so, would it have the same result?
The thing is that there's really no similar case. In 1972, perhaps Humphrey or Ted Kennedy would have been similar, but Humphrey didn't understand the new system and entered late, while Kennedy didn't run at all. In 1976 only Kennedy, who didn't run, might have been a similar heavyweight. In 1984 Mondale didn't really have the same kind of strength as Gore '00...he had never really run for president before, was four years out of office, and had been the VP for a failed president, not one who was wildly popular among Democrats. Kennedy, once again, didn't run. There's no one in 1988 or 1992 that comes close to qualifying. In 2004 there's Gore, who would have been a very interesting test, but he didn't run. In 2008, perhaps Gore still might have had field-clearing strength, but again he didn't run.
In other words, probably the second-strongest candidate to enter the Democratic primaries during the modern era was Walter Mondale in 1984, and he failed to clear the field. But that's in the early days of the reformed process, when it was far less clear how party actors could control nominations (in fact, 1984 is generally accepted as a major turning point).
It's perhaps also worth peeking over to the GOP side: in 1988, George H.W. Bush totally failed to clear the field, despite being if anything in a stronger position than Gore in 2000.
What I think all of this history says is that there are very few precedents, and those few we have don't really point in the same direction. And so...maybe.