Two major Democratic political groups on Wednesday announced a combined $21 million digital ad buy targeting Senate races in November, a sign the party is trying to learn from 2016, when Donald Trump’s Republican presidential campaign was far more aggressive online.

Priorities USA and Senate Majority PAC announced $18 million in joint spending in Arizona, Indiana, Florida, Missouri and North Dakota. Senate Majority PAC also tacked on an additional $3 million in ads targeting Montana, Nevada, Tennessee and West Virginia.

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“For the last really six years, the Democrats have had their hats handed to them when it comes to digital,” Guy Cecil, the chairman of Priorities USA, which is exclusively funding digital ads and outreach this election cycle, said in an interview. “We needed to close the gap.”

The move comes as Democrats and Republicans are fighting furiously over control of the Senate, where the GOP currently has a narrow 51-49 edge. Although almost all competitive seats are in states Trump won in 2016, Republicans are increasingly alarmed about the strength of Democratic candidates in states including Tennessee, Texas and Arizona.

The size of the campaign is significant — according to Priorities USA, $7 million has been spent on advertising for Senate races on Google since May 31, with Republicans outspending Democrats 60-40. Facebook did not have comparable data. And through the end of August, Senate Majority PAC, one of the biggest Democratic financial organizations in the battle over control of the upper chamber, spent $37 million in ads on television and radio, according to the Center for Responsive Politics.

J.B. Poersch, president of Senate Majority PAC, said it’s important to maintain a mix of traditional television and radio ads along with digital. But for years, he said, “I don’t think we had digital at the adults table.”

The conventional wisdom in politics is that Democrats dominated in digital during much of the Obama years because they were more advanced in gathering online data and using it to target voters. But that changed in 2016, when the Trump campaign outspent Hillary Clinton’s Democratic campaign nearly 2-to-1 online, according to a Priorities USA presentation to donors obtained by The Associated Press. The outspending also stretched to various House races. Right-leaning groups, meanwhile, registered vastly more online domains through the beginning of 2017.

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Since 2016, Democrats have increasingly focused on digital as a way to strike back against the GOP, with liberal Silicon Valley entrepreneurs holding trainings for Democratic campaigns and some liberal insurgent candidates, like Alexandra Ocasio-Cortez in New York City and Ayanna Pressley in Boston, winning recent primaries with minimal television ads and instead relying mostly on digital ones.

The GOP continues to invest in both digital and traditional advertising, but no Republican organization of comparable prominence to Priorities has announced an all-digital strategy. Priorities has even formed its own in-house digital ad agency to build spots for its campaigns, including a previously announced $12 million buy targeting House races.

Damon McCoy, a New York University professor who analyzed Facebook political ad spending data earlier this summer, said Democratic and Republican groups spend at comparable rates on the platform with one significant exception: Trump. The president’s own re-election campaign was the biggest political ad spender in the analysis that McCoy and other academics conducted.

“Removing him (and) the spending is fairly split between liberal and conservative candidates and political organizations,” McCoy said.