IOWA CITY, Iowa—Bernie Sanders may be falling behind some of his rivals in the crowded Democratic presidential field, dipping in early polls and seeing his campaign outraised by at least two rivals. But this week, he opened a new campaign office here and is already investing in states holding primaries eight months away.

The message from the Vermont senator: I’m not going anywhere.

Mr. Sanders, who rose to national prominence during his unsuccessful 2016 primary campaign against Hillary Clinton, has a fiercely loyal following and a formidable small-dollar fundraising operation that could allow him to last late into the 2020 contest—even if he doesn’t attract broader support in the polls. Because states will hand out delegates on a proportional rather than winner-take-all basis, Mr. Sanders could even have a path to the Democratic nomination with few outright primary wins if the field remains crowded and he keeps accumulating delegates over time.

So far, Mr. Sanders’s strategy has been to bank on his credentials as the original high-profile proponent of many liberal policies now championed by others in the race.

In 2016, Mr. Sanders advocated free college, a single-payer health care system and a $15 minimum wage. This election cycle, many candidates are promoting variations of those ideas, including Sens. Elizabeth Warren and Kamala Harris, who have surpassed Mr. Sanders in some early polls.