Sen. Bernie Sanders (I-Vt.) on Thursday became the first 2020 contender to announce reaching 1 million individual donors, according to Politico.

Why it matters: Donor sourcing is a hot-button issue for Democrats, with several progressive candidates opting to only accept grassroots donations during the primary. Sanders' campaign staff said he pledged not to accept high-dollar contributions into the general election as well, should he get the Party nomination.

The campaign said 99% of donors are eligible to donate again since the vast majority have not contributed the maximum allowable amount. More than 125,000 donors are giving monthly contributions on a recurring basis.

Sen. Elizabeth Warren hit the 1 million donation milestone in June, but has not yet announced 1 million individual donors to her campaign.

Go deeper: Bernie Sanders on the issues, in under 500 words