CEDAR RAPIDS, Iowa — The last time Senator Elizabeth Warren trundled across Iowa in a specially decorated campaign vehicle, an R.V. headed to the Iowa State Fair in August, it was wrapped with a cheeky play on the slogan for her sweeping agenda: “Honk if you’re ready for big, structural change!”

Now, with days before the Iowa caucuses, her bus is plastered with a far more direct and urgent message, as she looks to quell the skepticism that has slowed her momentum here in the crucial final stretch. “Courage over cynicism” it urges on one side; “hope over fear” it says on the other.

Such is the reality of her political comedown from the swaggering heights of the summer and early fall, when she was thought to be the standard-bearer of the Democratic Party’s left flank and the chief rival to former Vice President Joseph R. Biden Jr. These days, it is Senator Bernie Sanders of Vermont who is the liberal topping some Iowa polls and the top rival to Mr. Biden. Ms. Warren, of Massachusetts, along with Pete Buttigieg, the former mayor of South Bend, Ind., is battling to depart Des Moines next week with a mandate as a serious contender, leaning on a political organization she began building here earlier than her rivals.

She and her team have only a few days left to achieve, if not a win in Iowa, then a surprisingly strong finish that gives her momentum heading into New Hampshire. The campaign has said it is in the race for the long haul, warning in a memo last week against “breathless media narratives” that take hold as voting begins. The memo, written by Ms. Warren’s campaign manager, Roger Lau, touted a 1,000-strong staff spread across the country, including in the Super Tuesday states that vote in March. Still, the team now finds itself in the slightly surreal position of working flat out with no real sense of what 12 months of campaigning in Iowa will yield them.