The threat to democracy that the Trump candidacy represents does not exist because of his offensive personality. The real problem is that he has successfully tapped into the anger and alienation that exists widely in the country.
The base of Trump's support is non-Hispanic white men and women who are more than 45 years old, and who have attained no more than a high school education.  If the catastrophe of a Trump presidency is to be avoided, it is these people who have to be persuaded to direct their activism toward a more progressive agenda.
The years between 1999 and 2013 have been very difficult ones for that segment of the population. Adjusted for inflation, the weekly earnings of non-Hispanic white men declined significantly: a 9.6 percent decline for those with less than a high school education, and an 8.7 percent decline for those possessing a high school degree. This trend was not as unfavorable for non-Hispanic white women. Their weekly earnings, though very low, remained about the same during this period - in the neighborhood of $250 per week for high school graduates, and $175 for less well-educated women. 
One indication of the pressure this population is under was revealed by an important recent study by Anne Case and Angus Deaton. They showed that while death rates continued their long-term decline for all other demographic groups, mortality rates actually increased for white non-Hispanic Americans between 1999 to 2013. Most of this increase was due to stress-related causes of death, such as suicides and drug and alcohol poisonings. Case and Deaton only provide mortality rates by level of education for those between the ages of 45 and 54. But what those limited data show is consistent with the argument that dangerous levels of stress are concentrated among those Americans with the least education. According to the study, all of the increase in death rates that occurred did so among those with a high school education or less. Their increasing death rates stand in stark contrast to the continuing decline in death rates experienced by those possessing at least a B.A. degree. 
Case and Deaton speculate that there may be ties between "the epidemic of pain, suicide and drug overdoses" and "economic insecurity." In that regard, they point to the shift from pension plans with guaranteed payments to plans that are exposed to risks associated with the stock market. But the problem is almost certainly broader than that. Large numbers of Americans with less than a college education have no pension plans at all. What they do have, however, are declining incomes. The resulting economic insecurity has clearly taken its toll on their health; it may also have helped to produce a future President Trump.
There is nothing inevitable about the deteriorating conditions in which less well-educated Americans have found themselves. It is not as if the economy failed to grow during those years. In fact, between 1999 and 2014 the country's national income, adjusted for inflation, increased by 27 percent. However, none of this increase was shared with the non-Hispanic white population (or others) who possess only limited education - clear evidence of just how unfair our economic and political system is. The list of policies benefiting the rich, at the expense of the people with whom Case and Deaton are concerned, is a long one - the direct consequence of self-interested wealthy people funding political campaigns. Their political power has meant that trade unions have been crushed; that workers displaced by technological change and imports have been denied generous support and job training; and that literally thousands of blue collar jobs that could have resulted from a federal government program to rehabilitate the country's infrastructure were never created.
The irony is that many Trump supporters actually possess progressive views. The 51 percent of likely Republican voters who favor raising taxes on individuals who earn more than $200,000 a year overwhelmingly support Trump over other Republican candidates. The same pattern of preferring Trump exists among those who support labor unions, who favor raising the minimum wage, and who want a single payer health care system.  These are not conservatives. They are people who are furious at an unresponsive political system that is rigged against them.
To defeat Trump if he becomes the Republican presidential nominee, whoever runs against him will have to attract at least some of his base. This can only be done if he or she speaks to the political anger of Trump's supporters. To do so will necessitate a strong commitment to reducing the power of wealth in politics. Without that policy commitment, we all may have to deal with a dangerous demagogue in office for the next four years.
 David Bradly and Douglas Rivers, "Decoding Trump's Supporters", Hoover Institution, Defining Ideas, September 15, 2015. http://www.hoover.org/research/decoding-trumps-supporters
 Calculated from Bureau of Labor Statistics, Earnings by Education, "News Release: Usual Weekly Earnings of Wage and Salary Workers (archives)," 1999 and 2013. www.bls.gov/cps/earnings.htm#education. Consumer Price Index, 1982-84 = 100, Bureau of Labor Statistics, Consumer Price Index, "US City Average." http://www.bls.gov/regions/west/data/consumerpriceindex_us_table.pdf
 Anne Case and Angus Deaton, "Rising Morbidity and Mortality in Midlife Among White Non-Hispanic Americans in the 21st Century," Proceedings of the National Academy of Sciences, Vol. 112, no. 49, p. 7. For changes in mortality rates by level of education see Table 1, "Changes in mortality rate 2013-1999, ages 45-54." www.pnas.org/content/112/49/15078.full
 Michael Pollard and Joshua Mendelsohn, 2016 RAND Presidential Election Panel Survey, http://www.rand.org/blog/2016/01/rand-kicks-off-2016-presidential-election-panel-survey.html