Sen. Elizabeth Warren built a political career on her reputation as a financial expert. Her prospective presidential opponents are learning how savvy she is, at least when it comes to campaign cash.

Political observers spent this week wondering why the Massachusetts senator rushed to become the first major Democrat to announce for 2020. She made her strategy clear in a Wednesday-night interview with MSNBC’s Rachel Maddow. “This is going to be the fish-or-cut-bait year for the Democrats,” said Ms. Warren, in what was clearly...