“Successful fund-raising is one of the messages that you want to have available to you as a candidate in Iowa, because it’s an indicator of the ability to sustain a campaign as long as is necessary,” said Jerry Crawford, who has led several Democrats’ Iowa operations.

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The exact timing of Ms. Warren’s announcement — Dec. 31 — is significant. It means that in a few weeks, she will have to report her fund-raising totals for the fourth quarter of 2018, which will reveal how much she received on Day 1 in small-donor contributions — a key benchmark for popularity for Democrats in the nominating race.

Like Ms. Warren, candidates often announce their campaigns at the end or the beginning of a quarter, and the choice is strategic: If you announce at the end of a quarter and raise a lot of money very quickly, you can report an impressive total right away and help cement yourself as a serious candidate. If you announce near the beginning of a quarter, you give yourself almost three months to build a respectable total before you have to disclose it.

Message, test, message again

Another benefit of a full calendar year of campaigning is that it provides candidates with ample opportunity to test, refine or popularize new messages. Some Democrats already have signature taglines: Ms. Warren’s “Nevertheless, we persist,” or the Ohio Senator Sherrod Brown’s “the dignity of work.” Other candidates will be less decided and may try different messages to see what works.

Laura Belin, a close Iowa caucus observer and author on the Bleeding Heartland blog, said it was important for candidates to make residents feel as if they are being courted and their concerns are being heard. This could mean tailoring a national message for a local audience, or being responsive to a particular moment that resonates with audiences.