"RED FLAG!! RED FLAG!!! Clinton is only winning in Republican states! It's all over, all over! Trump wins!!"
That's the Ann Coulter/bullshit analysis you are hearing today. It's never clearly elucidated why it's bad for a Democratic candidate to appeal to conservative/moderate Democrats. Nor why this will lead to defeat in November. You are just supposed to believe it.
So let's try to apply some logic here. The argument seems to be that Clinton is appealing only to conservative/moderate Democrats, while Bernie Sanders is appealing to moderate/liberal Democrats. What impact could such an appeal have in the general election? Well, the first is that Potential Nominee Clinton would pull much larger votes in reliably "red" states. She would have no chance of flipping these states, so she would be "wasting" votes there. In reliably "blue" states, Clinton would so alienate the Democratic base that voters would stay home. This would flip several of these states Republican (say, Ohio or Florida).
Why is this nonsense?
Let's look at those "swing" states like Florida and Ohio. These are generally conservative states to begin with, and have voted Democratic in the last two elections only because the moderate Democratic nominee, Barack Obama, and the recession encouraged them to move into the Democratic column. Previously, both states strayed Democratic in 2000 (when the moderate Al Gore ran) and during the Bill Clinton administration (e.g., the candidacies of a center-right Southerner). The "Clinton is in trouble" argument assumes that only a rock-ribbed liberal like Bernie Sanders could hold these states. But there's little electoral evidence that Sanders would hold strong appeal in either state. Conversely, the "Clinton is in trouble" argument assumes that Republicans in both states will love Donald Trump and vote overwhelmingly and enthusiastically for him. That assumption is completely unfounded: What evidence we have indicates that a majority of Republicans actually dislike Trump. He may be the nominee but will be so offensive to voters that he'll lose these states (and others).
Let's take the "Clinton is in trouble" argument even further. Does Clinton "waste" votes in reliably Republican states? Actually, the contrary is true: "Wasted" votes are those where the winning candidate has already won the state but continues to build up support there. For example, Democratic presidential candidates handily win states like New York and Massachusetts. For a Democratic candidate to keep piling up votes in New York -- when those votes are needed in Ohio, Florida, or the Southwest -- is a "wasted vote".
Votes which a Democratic adds in a reliably Republican state are called "putting the state in play". This requires the Republican nominee to spend money and time defending his (Fiorina is out) base, rather than attacking the Democrat's base. These are not "wasted votes" but rather incredibly important votes which any Democrat would love to have. Indeed, in certain states (Ohio, Florida, New Mexico, Arizon, Michigan, Minnesota, Wisconsin, Illinois, Indiana, Virginia), a Democrat who begins to add votes there puts these states in play -- and in the case of Virginia, Florida, and Ohio, can conceivably flip the state Democratic (because these are swing states).
Is it possible that Potential Nominee Clinton could lose Democratic states? That's part of the "Clinton is in trouble" argument. The idea here is that Clinton will so alienate the Democratic base that Democratic votes will stay home, putting these states in play for the Republican nominee to pluck. This idea assumes that (a) Democratic voters are overwhelmingly liberal (unlike Clinton) and (b) that Clinton is so repugnant to them (e.g., she is the Trump of the Democratic Party) that they will stay home rather than vote against Trump. Neither assumption seems to hold water. To assume that the "overwhelming" number of Democrats are rock-ribbed liberals is not borne out by surveys, and Clinton so far has 2.873 million votes in Democratic primaries compared to Sanders' 1.585 million votes. So far, the evidence indicates that two-thirds of the Democratic base is moderate/conservative. As for the second assumption, in only a small number of cases (Dukakis, Mondale, Carter in 1980) have Democratic voters stayed home due to a nominee they hated, and in all these cases the effect has been small. We have much stronger electoral evidence for voters "coming home" -- that is, holding their nose and voting Democratic, because they just can't stand the GOP.
So this "RED FLAGS!!! CLINTON ONLY WINS IN REPBULICAN STATES!! FLEE!! FLEE FOR YOUR LIVES!!!!!!!" argument is just bullshit.