"If the numbers don't pick up, we run the risk of having to pull back plans to organise for Elizabeth Warren in all 50 states during the primary", the e-mail said, according to a report in the Boston Globe.
The campaign asked supporters for more funding before 31 December and the end of this year, the report said.
According to the email, Warren's campaign acknowledged that it only raised $17 million in the last quarter of 2019 compared with $24.6 million in the third quarter - a drop of $7.6 million, or 31 percent.
Warren continues to be listed in all opinion polls among the top three Democratic presidential candidates.
Still, she has failed to cut significantly into former Vice President Joe Biden's support base and has lost ground significantly to Senator Bernie Sanders' energetic campaigning, the polls say.
Warren, a 70-year old Democrat and fierce critic of President Trump, comes from the so-called ''progressive wing'' of the party and is mostly known for her "Medicare for All" healthcare plan. Her healthcare policies, however, have been criticised by both Democrats and Republicans. This year, the 2020 candidate also drew attention after she was caught lying about her alleged Native American origins.