Democratic small-dollar donors gave $1 billion through the online fundraising platform ActBlue in 2019,officials said Thursday, highlighting the explosion of online giving on the left heading into the presidential election year.

Of the 6 million donors who gave to Democratic candidates and organizations in 2019, half were first-time donors, officials said, pointing to the growing base of contributors who are giving online. Forty percent of the new donors gave multiple times, according to ActBlue, in a sign of the new donors’ sustained political interest and engagement.

Donors contributing in low increments online gave $343 million in the final three months of 2019, officials said.

The surge in online low-dollar donations points to the enthusiasm among Democratic voters and the increasing sophistication of campaigns and organizations in reaching donors on digital platforms and making it convenient for them to give money.

More than half of the donations on ActBlue in 2019 were made on mobile, marking the first year the majority of donations came through mobile devices, they said.

The Republican Party also stepped up its small-dollar online fundraising efforts in 2019 with the launch of WinRed, a donation platform modeled after ActBlue.

WinRed announced this week that it had raised $101 million since it launched in the summer of 2019, with almost $70 million in the final three months of the year.

WinRed officials credited House Democrats’ impeachment of President Trump for a surge of online donations on the platform in the final months of the year, thanks to donors showing their support for the president and the Republican Party amid the impeachment inquiry and vote.

All Republican state parties and the majority of GOP Senate and House campaign committees have now adopted WinRed, officials said.

Small-dollar donors have played an important role during the Democratic presidential primaries in 2019, reshaping how candidates raise money in order to remain viable.

Erin Hill, ActBlue’s executive director, said in a statement that the record-breaking Democratic online donations are a positive sign for the party and for its eventual presidential nominee, who will face Trump’s formidable small-dollar online fundraising machine.

“Our nominee will need at least half of their funds from grassroots donors if they want to beat Donald Trump. Based on what we saw last year, the eventual Democratic nominee will have an army of grassroots donors behind them,” Hill said.

The number of donors and individual contributions were higher in 2019 than in any other previous year since ActBlue’s founding in 2004, officials said.

In 2019, more than 13,000 candidates and causes received donations through ActBlue, totaling 35 million contributions, officials said. The average contribution in 2019 was $30.50, they said.

As candidates and groups made a final push for donations at the end of the year, Dec. 31 resulted in the biggest fundraising day of 2019 on ActBlue in terms of the amount of money raised — more than $20 million — and the number of contributions, 525,000, officials said.

Donations made through ActBlue and WinRed will be revealed Jan. 31 in public reports filed with the Federal Election Commission.