After making Facebook advertisements a key part of his 2016 run, President Donald Trump’s campaign continues to spend heavily on the social network as Democratic presidential candidates begin their own social media efforts.

Trump spent $3.6 million on Facebook ads from Dec. 30 to March 23, according to data compiled by researchers at the Online Advertising Transparency Project, an ongoing study launched last year at New York University that tracks political ad spending through Facebook’s publicly available political ad archive.

Trump’s spending in early 2019 was more than the Democratic campaigns combined, but he has been out-spent by some candidates in recent weeks. Former Texas Rep. Beto O'Rourke out-spent Trump two weeks ago, dropping $157,000 on Facebook compared to Trump’s $109,000. Last week, Sen. Amy Klobuchar, D-Minn., spent $198,000 to the president’s $150,000.

The data provides fresh insight into the themes and strategies that the Trump campaign is employing in the earliest days of the 2020 election cycle. Facebook launched its political ad archive in May 2018 amid mounting concerns about Russian interference in the 2016 presidential election, an effort that included the purchase of thousands of divisive Facebook and Instagram ads by Kremlin-backed agents.

Trump’s ads have primarily focused on his political opponents and his efforts to build a wall along the southern border.

Among Trump’s biggest targets in the Democratic Party have been Senate Minority Leader Chuck Schumer, D-N.Y., and House Speaker Nancy Pelosi, D-Calif. Trump’s ads have mentioned those two lawmakers in at least 16,300 targeted Facebook ads since January, almost 40 percent of the total, according to an NBC News analysis of the data.

A screenshot from Facebook's political ad archive showing one of the Trump campaign's ads and data about the ad buy. Facebook Screenshot

The wall was the main subject of more than 30 percent of the president’s ads during this time period, while appeals for more general support made up 24 percent of his ads. The State of the Union address (15 percent), contests for meals with the president (14 percent) and attacks on the “mainstream media” (7 percent) followed as the most popular talking points for Trump ads.

In hundreds of ads — featuring captions like “How much more American blood must we shed before Congress does its job?” — Trump urged supporters to contribute to an “Official Secure the Border Fund,” donations that went to the president’s re-election effort.

Trump bucked tradition by announcing his 2020 candidacy on the day of his 2017 inauguration. Since then, he’s raised more than $100 million, a historic rate for a presidential campaign. Trump spent heavily on Facebook in the run-up to the 2016 election, buying more than 5.9 million ads, Trump’s 2020 re-election campaign manager Brad Parscale told PBS.

Trump’s Facebook ad buys come from two groups: Donald J. Trump for President Inc., the official campaign committee, and the Trump Make America Great Again Committee, a joint fundraising committee composed of the Trump campaign and the Republican National Committee. Those groups spent about $1.3 million in the first two full weeks in January, with spending declining in recent weeks, the data showed.

Between Jan. 6 and mid-March, Trump’s campaign made more than 42,000 distinct ad buys. Most of those were the same ads with different target audiences, according to Laura Edelson, a doctoral candidate and member of NYU’s Online Advertising Transparency Project.

Trump’s Facebook ads are also responsive to the news cycle. On Tuesday, the Trump Make America Great Again Committee launched 14 versions of a new Facebook ad that referred to the end of special counsel Robert Mueller’s investigation.