The thing about Presidential elections was that to give states more input into the voting process, the Founders designed the election to matter by how many states you won rather than the whole overall popular. That way, the candidates wouldn't have to pander to just a handful of big-population states, they'd have to pander to a few of the smaller states to cross the finish line.
This can still cause some problems. Say for example, one candidate goes and wins over every voter in California at 38 million voters while the opponent gets most of the remaining states but at slim leads, leading to the first candidate getting 62 million total voters to the second candidate losing the popular vote at 54 million... yet winning all 49 states gives that second candidate ALL those Electoral votes for a decisive win. It's been a problem due for major reforms for a long time, even before the debacle of 2000 that showed the College's flaws.
Nonetheless, it's the system we have, so the parties play to the states.
Which is why the candidates tend to behave the way they do. First and foremost, they need to confirm the states that are already in their pockets.
The Democrats are going to ensure that the states who voted for Obama in 2012 remain to their side in 2016: as long as they secure those states they already win. Getting more states to turn Blue would be a bonus (and generate stronger argument about a Mandate). As a result you're likely going to see a more cautious, pragmatic, moderate-toned campaign.
Republicans have to be more aggressive: they have to turn about three or five major Blue states to their side to secure a win, which means they need the likes of Florida, Ohio, Pennsylvania, Wisconsin and Virginia to turn. But at the same time they can't lose ANY states already Red, which is possible in places like Georgia, Arizona, North Carolina and Texas where the demographics are inching further Left. That aggressive campaigning is going to involve a lot of Dem-bashing, because negative campaigning to reduce voter interest is a sadly effective means of getting states on or near Toss-Up standings to tilt your way.
Here's what 270towin has as their Toss-Up map right now, where the colored states are "confirmed safe" for the appropriate Party:
I am a bit surprised that the likes of Colorado and Pennsylvania would be toss-ups: there's a sizable Democratic contingent in those states with more urban, Left-leaning voting blocs. I'd have Georgia more of a Toss-Up state due to the demographics (state is skewing younger and more professional as Atlanta grows as a major urban market).
I'm not surprised Florida is a Toss-Up: God knows which way the voters will act this year.
In my humble opinion, right now the states Obama won in 2012 are about as safe for Democrats as could be. There is a risk that the wrong candidate (Hillary in certain cases, Sanders in others) could depress Dem voter turnout. However, the way the Republicans are acting right now in the Primaries, I can't see that many moderate/independent voters switching to Red. I just can't.
And if it's Trump as the main candidate for the Republicans - with his anti-immigrant, Mexican-bashing platform - I honestly see states like Florida and Colorado staying firmly Blue, with the likelihood of places like Arizona and Texas switching parties (if the Republicans lose Texas, they lose the whole match because the only other big state to balance is California... and the Hispanics there still haven't forgotten what the Republicans did in the 90s).
So if the election in 2016 ends up being, oh, Hillary vs. Trump, the map is gonna look like this:
Granted, I'm not polling people or a full-bore political pundit expert living this stuff 24/7, but I'm setting this as a complete rejection of the Republican Party if Trump's anti-Hispanic campaign plays all the way into November. As many Mexicans and Hispanics in states like Arizona, Nevada, Colorado, Texas, Florida and Georgia are going to go full Dem much like Black voters have done since 2008. I've got UTAH switching Blue because the Mormons are doing a lot of outreach and community-building among Latinos, and even they can turn against the Republicans on this issue despite being very social-conservative on other issues. I don't see any 2012 Blue state turning Red because of Trump.
I have Kansas, North Carolina, and Louisiana turning Blue because of intrastate controversies revolving around corrupt or inept state Republicans mismanaging those states.
In my prediction, Hillary wins against Trump with 432 Electoral votes to his 106 (!), with 34 states backing her. It'd be one of the biggest Democratic wins since LBJ pummeled Goldwater in 1964.
Against any other Republican, as long as that Republican candidate pulls back from a harsh anti-immgrant stance, Hillary might have a harder fight. Say for example, it's Hillary vs. Ted Cruz:
Cruz's outreach to the Evangelicals will net him Virginia and Iowa back (there's a sizable Christianist population in those places) but that's about it.
And if it's Sanders as the Democratic candidate? Against Trump, it'd be the same map as Hillary's: Trump's anti-immigrant stance is that destructive. Against Cruz, it'd look a bit like this:
Sanders' Far-Left stance - his push for universal health-care especially - isn't going to win over a lot of Red states where "socialism" is still a four-letter word. More conservative-leaning Blue states might switch. Florida becomes a battleground state again: it could go either way (I have it barely staying Blue thanks to enough emigre voters from the North making it more Purple than Red).
Once a clearer picture of the Republican race is made, and an obvious candidate in the forefront, we'll have a better idea how the Electoral race is going to turn out.
Of course, the key thing for Democrats is GET THE DAMN VOTE OUT. Elections matter.