It's easier when your candidate doesn't get railroaded by the DNC. Don't run trash candidates, and people will vote.
"Vermont Sen. Bernie Sanders (I) is the country’s most popular active politician, underscoring his importance to the Democratic Party as it seeks to rebuild in the wake of a disastrous 2016 election cycle.
Sanders is viewed favorably by 57 percent of registered voters, according to data from a Harvard-Harris survey provided exclusively to The Hill. Sanders is the only person in a field of 16 Trump administration officials or congressional leaders included in the survey who is viewed favorably by a majority of those polled."
http://thehill.com/homenews/campaign/329404-poll-bernie-sand...
"Former Democratic presidential nominee Hillary Clinton's favorability rating is at a new low one year after her election loss to President Trump, according to a poll released Tuesday.
Clinton now holds a 36 percent approval rating among Americans, according to Gallup, down 5 percentage points since June. The rating falls below Clinton's previous low of 38 percent in August to September of last year.
The former first lady also reached a new high disapproval rating of 61 percent. Clinton has bucked the trend of defeated presidential candidates gaining popularity after the election, Gallup says."
http://thehill.com/blogs/blog-briefing-room/365656-gallup-hi...
Trump's approval rating (41%) is still above Clinton's; his disapproval rating below hers as well. That says quite a bit.
https://projects.fivethirtyeight.com/trump-approval-ratings/
And finally, US support for universal healthcare:
"A majority of Americans say it is the federal government’s responsibility to make sure all Americans have health care coverage. And a growing share now supports a “single payer” approach to health insurance, according to a new national survey by Pew Research Center.
Currently, 60% say the federal government is responsible for ensuring health care coverage for all Americans, while 39% say this is not the government’s responsibility. These views are unchanged from January, but the share saying health coverage is a government responsibility remains at its highest level in nearly a decade."
http://www.pewresearch.org/fact-tank/2017/06/23/public-suppo...
Sorry for the wall of text, there was nothing I could cull away without removing data points to support the argument.