Mayor Setti Warren of Newton has made calls to Democratic donors, asking for campaign contributions and saying he will challenge Governor Charlie Baker in 2018, two people who spoke with him said.

The move makes Warren the first Democrat across the starting line in the still-nebulous field to chase Baker. It also gives Warren a fund-raising advantage by allowing him to meet the annual state limits of three separate calendar years.

One person who spoke with Warren late Tuesday said he asked for donations of between $500 and $1,000 by the end of 2016.

“ ‘I am going to run for governor and I am starting my campaign now’,’’ said the person Warren called, relaying a private conversation on the condition of anonymity.

A second person who spoke with Warren confirmed a similar conversation.

Despite announcing this month that he would not seek another term, Warren is holding a Dec. 6 fund-raiser at the Boston law firm Foley Hoag, according to an invitation obtained by the Globe.

Warren has also hired longtime Democratic organizer John Walsh, a top adviser to former governor Deval Patrick and onetime state party chairman, in a move first reported by Politico.

All of those moves signaled a likely 2018 bid, but the round of calls specifically asking for financial assistance provided the most concrete evidence yet.

Warren first won the office in 2009. He won praise from residents and fiscal watchdogs for putting the city on a financially sustainable course.

Joshua Miller of the Globe staff contributed to this report. Jim O’Sullivan can be reached at jim.osullivan@globe.com.