Dead-ender: A partisan who refuses to stop supporting a cause or candidate; a die-hard.
This sense of dead-ender – the most recent of several to have appeared during the term’s history – has appeared regularly during the current U.S. presidential campaign. It’s used as a disparaging tag for zealous supporters of candidates whose realistic chances of prevailing have steadily dwindled. These supporters often self-identify on social media with the hashtag #NeverHillary or #NeverTrump.