Ms. Gabbard has disputed the polls selected by the national committee as “certifying” candidates for the debate, arguing that many of the noncertified surveys are more accurate. Those polls could also help Ms. Gabbard qualify for the November debate.

Her call won support from a fellow primary candidate, Marianne Williamson, who is also polling among the bottom tier of presidential hopefuls.

“I have great respect for Tulsi for saying such inconvenient truth,” Ms. Williamson posted on Twitter last week, after Ms. Gabbard first raised the idea of boycotting the debate.

Ms. Gabbard’s warnings of a rigged election are likely to resonate with her base, an unconventional mix of anti-interventionist progressives, libertarians, contrarian culture-war skeptics, white nationalists and conspiracy theorists. They like her isolationist foreign policy, her calling out of what she sees as censorship in the major technology platforms and her support for drug decriminalization.