Lets take of 2016 before we measure the Oval Office for 2020 drapes.

The whole landscape could be very different in our volatile and changing times where more and more people will feel the need to pressure for big changes then. I think it's not very healthy to measure what will happen then, when we don't know yet how Hillary Clinton will campaign to those who want real constructive change, and then how well she will in fact deliver on those promises (which arguably many people are disappointed in Obama setting a bad example of not living up to the "Hope and Change" people were inspired to vote for).

We can't answer this sort of question until whoever gets elected gets elected, and we've had some time for the government lead by that president to enacting substantive and useful changes. Just talking this way will make many out there feel even worse if they feel a lot of this sort of talk is unheard later and not delivered on.

In short, many voters now are thinking that old line of... "Promises, promises!..."

Ernst will be at most a one term senator, she has several scandals brewing back home. I would not be surprised if she resigns before her first term is up.
Regarding Hillary, I see her winning easily in 2016. Hillary will make a truly great president but I hope she will only serve one term so her pick for vp will be especially critical as they will be the heir apparent.
If she picks a Hispanic such as Julian Castro the 2020 race will be an easy win for us, the democrats, because of changing demographics. By 2020 many of the dreamers will be eligible to vote along with millions of other peoples of Hispanic origin who now have amnesty. President Obama's decision to grant executive amnesty will be seen as the beginning of a permanent Democratic majority nationwide much like the permanent majority Democrats now enjoy in California.
Once we have a permanent Democratic majority the courts will become ours. Once the courts are ours districts will be drawn to favor the majority. Decisions all the way up to and including the Supreme Court will favor our views, for instance the 2nd ammendment will finally be interpreted as written and our country will join the rest of the civilized world in controlling firearms.
The key to this is Hillary must serve only one term, if she is the historical figure I imagine she is she will see her part as a piece of the big picture, the transformation of the United States into a more fair and perfect union along with the permanent ruin of the Republican Party.
If the republicans cannot defeat Hillary and I don't see any way they can, they are looking at their demise. I think deep down they know this, you won't hear them say it, the words are too frightening for them to speak, but it's the truth.
They are going to throw everything they have against Hillary, it's going to get ugly. We saw last week Rand Paul destroying himself with his sexist attack on Savannah Guthrie, the same thing will happen to the others if and when they attack Hillary. Women will see men at their worst, nothing but bullies picking on a fellow woman, men (real men not repuke pigs) will see them as weak and pathetic sissies who gang up to attack the stronger and smarter woman. It's a lose-lose for those running on the other side. They don't know it yet but there is no way they can win this election.

And they barely won 2004 (by what, 51% of the popular vote right?), and needed the SCOTUS to give them the White House in 2000, 2008 and 2012 were jokes. I think its safe to say that if Hillary wins in 2016, and she doesn't make a fool of herself, she could look forward to a 2020 win also. Hillary's pretty good at not looking like an idiot, so I doubt she'll step in it that badly.

The Republicans haven't fielded a serious candidate in 15 years, I'm counting this year so far. They've only got clowns except for Jeb. Rand Paul? Raffi Cruz? Rubio? Perry? Come on! All these guys are looney-toons, they'll play well in the deep south and nowhere else. If its not Jeb this year, then this 2016 election is Hillary's race to lose. If it is Jeb this year, it will be tough, but Hillary can pull it off. She'll seem more "fresh" than yet another Bush running the economy into the ground... again.

2000(Gore) and 2004(Kerry) were not as crazy as Dole,McCain,Romney or the 2016 Republican Presidential candidates.