I can think of two modern cases of "factional" candidates who emerged as serious threats to win nominations after the primaries began and who were then stopped when the party united behind an alternative: Jesse Jackson in the '88 Dem race and Pat Buchanan in the '96 GOP race.
Sanders is often compared to these two, for some understandable reasons. But a big difference I keep seeing is that Sanders -- unlike Jackson/Buchanan -- has favorable/unfavorable numbers among Dem voters that are as good/better as any other candidate.
There were also clear ideological/demographic limits that hindered Buchanan and Jackson as they tried to build on success. Sanders, though, is showing more potential than other non-Biden candidates to win significant non-white support and also registers among self-ID's moderates
When I look at Sanders' prospects among Dems, I'm reminded more of Trump in the '16 GOP race. There were lots of assumptions that he had a hard ceiling, but he was well-liked by R voters and his support spanned the ideological spectrum within the party. He was able to build.
