My answer is always the same: Hillary Clinton would start out as a favorite -- albeit not a heavy one -- over Jeb Bush or any other Republican due, in large part, to the built-in advantage Democrats currently enjoy in the electoral college. (That edge is built on demographic shifts in the country that have largely favored Democrats.) The GIF below -- built by @metricmaps -- shows how the electoral map has changed over the past three decades, transitioning from Republican dominance in the 1980s to Democratic superiority in 2008 and 2012.
The simple fact is that, in 2016, the Democratic candidate -- I assume that will be Hillary but it could be someone else and not make a huge difference -- starts at a significantly higher point in terms of electoral votes than whomever Republicans nominate.
Let's use President Obama's 2012 victory as the example. In that race President Obama won 332 electoral votes to 206 electoral votes for Mitt Romney. Here's what the map looked like:
Now, let's look at the 10 closest states -- by percentage -- in the race and subtract them from each man's electoral vote total. Those states -- Florida, Ohio, North Carolina, Virginia, Colorado, Pennsylvania, Iowa, New Hampshire, Nevada and Wisconsin -- comprise 130 electoral votes. Obama won all of them except for North Carolina and its 15 electoral votes. So, take 115 votes from Obama's column and 15 votes from Romney's column. That brings Obama to 217 electoral votes and Romney to 191 -- a built-in 26 vote electoral vote edge. That edge is actually deceivingly low because it excludes Pennsylvania and its 20 electoral votes from the Democratic side. And, while Pennsylvania was one of the 10 closest states in 2012 -- Obama won it by just over 5 percentage points -- the state hasn't voted for a Republican at the presidential level since 1988. Add Pennsylvania to Obama's total and he starts at a baseline of 237 electoral votes -- only 33 short of the 270 you need to get elected president.
And, those electoral college advantages are not unique to Obama's 2012 map. Remember that in 2012 he didn't carry Indiana (as he did in 2008) or North Carolina, and the the 237 total above doesn't include swing states like Ohio, Virginia Florida, Colorado and Iowa -- all of which Obama won in 2012. In fact, there's an argument to be made that the electoral map in 2016 -- regardless of the two candidates -- will be even more challenging for Republicans than the 2012 map. The growth of the Hispanic community in places like Arizona and Georgia means those states could teeter on being potentially competitive in 2016, and, if Republicans remain unable to win any significant swath of the Hispanic vote, will be prime battlegrounds in 2020 and beyond. Texas -- and its treasure trove of 38 electoral votes -- could follow suit in 2020 or 2024.
While demographic changes are moving a number of traditionally Republican states closer to Democrats, there's little evidence that many states are heading in the opposite direction. You could make the case that Wisconsin is moving closer to Republicans' grasp (it was the 10th closest state in 2012), and Minnesota -- the 11th closest state -- might be shifting ever-so-slightly in Republicans' direction as well. The problem is that big states like New York, California, Michigan and Pennsylvania show no signs of becoming more friendly toward Republicans; in the case of New York and California, they are becoming far less friendly to the GOP. With those major electoral vote targets off the table -- or close to it -- the math becomes increasingly difficult for Republicans.
Here's the lone comfort at the moment for Republicans: The electoral college tends to move like a pendulum. In 1980, Ronald Reagan won 489 electoral votes and followed that up four years later with 525 electoral votes. In 1988, George H.W. Bush took 426 electoral votes. The Republican lock on the electoral college seemed permanent. But then it wasn't anymore. It's not clear -- at least to me -- how Republicans will pick the Democratic lock on the electoral college but history suggests they will, eventually, find a way.