DES MOINES — The presidential race hurtled over the weekend toward a watershed moment: voting that will start to reveal the true depth of Americans’ desire to cast aside traditional politicians and Washington-style compromise and embrace disruptive outsiders appealing to their passions.

After a year of countless and often conflicting polls, more than 250,000 Iowans are expected to attend caucuses on a relatively mild Monday night and render judgment on insurgent candidates who would bar Muslims from the country (Donald J. Trump), oppose concessions to Democrats (Senator Ted Cruz of Texas) and pursue a high-tax, big-government agenda (Senator Bernie Sanders of Vermont). Voters are poised to bring order to the race, or reorder politics, as in no other recent election.

Money, experience and endorsements — advantages that usually turn candidates like Hillary Clinton and Jeb Bush, the former governor of Florida, into inevitable nominees — will be tested against the potent messages of rivals promising upheaval.

The importance of aggressive fund-raising and campaign commercials, which have cost a combined total of more than $100 million so far, will become suspect if the social-media-driven organizing by grass-roots groups helps yield upset victories for candidates like Mr. Sanders.