Comparing the 2016 race to the political comebacks of Richard Nixon and Ronald Reagan overstates the obstacles facing a contender who lost elections. Nixon prevailed during a very unpopular war with a promise of a secret peace plan. The incumbent party could not escape the specter of Vietnam in 1968. Reagan prevailed during heavy stagflation in the economy, with the inadvertent help of an independent candidate who split the vote in 1980. Public concerns over war and economic prosperity trump any other consideration in voters' minds when electing a President.
Elections in times of peace and prosperity can turn more on the personal characteristics of candidates but Americans still have strong preferences for incumbents. Charisma matters to voters who want a candidate they think will be just like themselves. Successful Presidential candidates are of course nothing like commoners, and their charm in appearing so is a useful mask. Ronald Reagan was everyone's favorite uncle or grandfather in 1984, depending on one's age group. The two former governors leading the GOP pack in 2015 remind a lot of working class Americans of their supervisors at work. The former First Lady on the Democratic side reminds a lot of Gen-X and Boomer professional women of themselves. I'm pretty sure that whichever party nominee picks a Hispanic running mate will lock up a lot of newly minted Nuevos Americanos who want someone just like themselves.
John Williams' Shadow Government Statistics published its Commentary #442 on May 12, 2012. It is noteworthy for its discussion of how real disposable income growth affects presidential elections. This figure was less than +3.0% in 2012 and yet the incumbent won reelection, bucking the historical trend since 1932. The substitution of large government entitlement programs for personal income is a recent phenomenon. Its power to mask weak income growth is phenomenal. Food assistance (EBT/SNAP), free cell phones, and SSI disability payments do not count as earned income but represent hidden purchasing power for low-income voters. The mentality of a candidate who tells private businesses "you didn't build that" reveals the attractiveness of handouts as an election year ploy.
The leading candidates of both parties at this stage of the 2016 campaign are notable for their bland public personas. The two former GOP governors who worked in finance appear stiff when they try to affect a common touch. Their private sector accomplishments are less impressive to low-information voters than how they might look wearing a plaid shirt and denim jeans. The leading Democrat offers the novelty of breaking the gender glass ceiling. Her fans are willing to overlook her lack of accomplishments in national office. It is fair to ask how many bills she authored in the US Senate and how many diplomatic problems she resolved as Secretary of State. Most people won't care about the answers, but history will note our indifference.
The two leading center-right Republicans share the same donor base; the one that raises the most money will probably win the nomination. The leading Democratic contender has a ready-made donor base in her husband's non-profit foundation. All three of them have longstanding friends on Wall Street. Whoever wins the fundraising race by mid-summer 2016 will own the messaging through November. The state of the economy in the summer and fall of 2016 will be the primary factor driving the electorate's final choice but funded messaging will matter only in swing states. Commitment to a large new war is unlikely between now and then, so that controversy will probably not be present in the race.
The GOP nominee from 2012 came close but got no cigar thanks to the incumbent's gifts to his base in swing states. Organizing for Action's (OFA) permanent campaign is a template for operatives who want to win. There is little indication in open sources that the GOP has mastered the geolocated Big Data that drove the 2012 policy handouts in swing states. The only outcome in doubt for the 2016 election cycle, in the absence of an economic crisis or difficult war, is whether the obvious front-runners assemble OFA-style campaigns.
Full disclosure: I voted for Mitt Romney in 2012, and I would like to do so again. His career experiences made him the best-qualified choice in 2012.