Here's why either Clinton or Sanders can beat Trump:
(1) A recent Wall Street Journal/NBC News poll revealed that the number of Americans who identify themselves as "liberals" is increasing while the number who call themselves "conservatives" is declining. (http://www.politicususa.com/...). I'm not a big fan of polls that ask people to identify themselves as liberals, conservatives, moderates, etc. What's more important is what people actually believe about specific issues and concerns about themselves, their families and their country. Most voters vote based on whether they think a candidate agrees with them on issues. Even if they are somewhat fuzzy about the candidates' actual policy ideas, they have a reasonably good sense of what candidates believe.
(2) Most Americans are liberals even if they don't identify as liberals. Even many self-described conservatives have so-called liberal views on many key issues. For example, about three-quarters (74%) of Americans—including 84% of Democrats, 72% of independents, and 62% of Republicans—believe that corporations have too much influence on American life and politics today. Sixty-eight percent of Americans favor raising taxes on people earning more than $1 million per year, including 87% of Democrats, 65% independents, and 53% of Republicans. Three quarters (75%) of Americans (including 53% of Republicans) support an increase in the federal minimum wage to $12.50 by 2020. Sixty-three percent of Americans support an even greater increase in the minimum wage to $15.00 by 2020. Polls show that Americans are generally progressive on economic issues. (http://prospect.org/...)
(3) These huge numbers in favor of progressive policies mean that many people who describe themselves as "moderates" and some who call themselves "conservatives" support many progressive policy ideas. And, as one analysis of recent polls noted, "Socially, the United States is moving to the left. Majorities in the country support immigration reform, same-sex marriage, a woman’s right to choose, and accept climate change as reality. On all of these issues, Republicans are in the minority." (http://www.politicususa.com/...)
(4) On the other side of the political divide, about 10% of Americans are hard-core right-wingers on all issues, including race and immigration. They are, like the core of NRA members, very vocal and political involved. They watch and agree with Fox News and listen to Rush Limbaugh. They tend to vote in Republican primaries. They fueled the Tea Party and they are fueling the Trump surge. But Trump has a ceiling of support and he is close to reaching it. In 2012, about 19 million people voted in the GOP primaries. If the same number vote in the 2016 GOP primaries, and Trump gets 25% of them, that's 4.7 million votes. That's less than 4% of the 129 million Americans who voted in the general election. I doubt Trump can win more than 35% of the general election vote (that's about 45 million votes) against any Democrat. The polls show Clinton beating Trump by a landslide in both the popular vote and the Electoral College vote. ( http://www.examiner.com/...)
(5) The one serious poll I've seen regarding a one-on-one match-up between Sanders vs Trump (conducted by CNN) shows Sanders winning decisively. Although it didn't calculate Electoral College voters, it appears Sanders would have a big edge there, too. (http://thehill.com/...). The size of Clinton's or Sanders' triumph over Trump in November 2016 would depend on the turnout by young voters, African American voters, women voters, and Latino voters.
(6) But I don't think Trump will win the GOP nomination. As I've written elsewhere, I'm reasonably confident that as the weaker GOP candidates drop out and Trump is left to compete with Bush, Rubio, and Kasich (or Walker), these other candidates will win the primary votes of those who had previously been for the GOP dropouts. The question is whether one of them can garner enough votes to trump Trump. (I still think Bush will prevail and he'll pick Kasich as his VP in order to win Ohio). But if not, and in the unlikely case that Trump wins the GOP nomination, he can't beat Clinton, Sanders, or even Biden.
(7) And if Trump drops out of the GOP race after, say, Super Tuesday, and runs a third party campaign, he'll take enough voters away from the Republican candidate to definitely elect a Democrat as president in November 2016. (http://www.salon.com/...)
Peter Dreier is professor of politics and chair of the Urban & Environmental Policy Department at Occidental College. His most recent book is The 100 Greatest Americans of the 20th Century: A Social Justice Hall of Fame.