Appearing on "Outnumbered Overtime" with host Harris Faulkner, the Fox News contributor said that Warren's numbers are dwindling.
"Not only has it stopped -- it's reversed," he said. "A month ago she was right at Biden's heels."
The former vice president has recently stepped up his attacks on the senator from Massachusetts, most recently releasing an op-ed accusing Warren of elitism after the release of her $52 trillion "Medicare-for-all" plan.
In an interview on SiriusXM's "Urban View" on Wednesday, Biden said that Warren: "has things in her plan that are just not realistic, but when you question it she talks about it as, 'Well, you just don't understand' or 'You're sounding like a Republican,' which is a talking point that is used by the other party, not by us, it's not who we are."
"And, it's just an elitist attitude about -- you're either my way or the highway and you mustn't be -- you must not know what you're talking about if you disagree with me," he added.
Stirewalt remarked that "it's rare that someone uses the contraction 'mustn't' when calling someone else an elitist, so I do appreciate that.
"The reality is, it's [her plan is] working against Warren," he said. "My advice to her for this [upcoming] debate is hydrate because it's going to be thirsty work up there."
Stirewalt told Faulkner that Democrats across the spectrum "have all agreed that Elizabeth Warren has failed."
"And, by the way, the pundit class, the rank and file Democrats ... Elizabeth Warren has had just a 'stankeroo' two weeks. It's been horrible for her," he added.
"She's in big trouble," he concluded.
With Warren set to slide, Stirewalt had a ready choice to take her place as the challenger to Biden.
"The one I'm watching? Pete Buttigieg."