Sen. Bernie Sanders Bernie SandersKenosha will be a good bellwether in 2020 Biden's fiscal program: What is the likely market impact? McConnell accuses Democrats of sowing division by 'downplaying progress' on election security MORE (I-Vt.) has ramped up spending on Facebook ads in states holding primaries this week, as he faces an increasingly steep climb to winning the Democratic presidential nomination.

The Vermont independent more than doubled Facebook spending by his Democratic rival, former Vice President Joe Biden Joe BidenSenate Republicans face tough decision on replacing Ginsburg What Senate Republicans have said about election-year Supreme Court vacancies Biden says Ginsburg successor should be picked by candidate who wins on Nov. 3 MORE, in Arizona, Florida, Illinois and Ohio from March 5-11, according to data compiled by crowdfunding technology platform Booster that was first shared with The Hill.

Ohio's governor on Monday called for postponing the state's primary elections until June due to the worsening coronavirus outbreak.

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Sanders spent $525,239 on Facebook advertising in the four states, capturing 67 percent of the spending between the two candidates. Biden spent a little more than $217,00 in those states.

Biden slightly outspent Sanders in overall Facebook ad spending, $1.25 million to Sanders’s $1.24 million. A significant chunk of Biden's ad budget has gone toward states that have already voted, suggesting his campaign is already shifting its focus to the general election.

For example, 12 percent of Biden's Facebook ad budget was focused on fundraising in California, which held its primary on March 3.

"The Biden campaign is already shifting a higher percentage of their Facebook advertising budget into fundraising with less of a focus on persuasion and [get-out-the-vote] ads as only 17 percent of Biden's Facebook ad budget over the last week was spent in states that vote [Tuesday]," Booster co-founder Aaron Earls told The Hill. "We can expect his campaign to shift its focus back toward using Facebook ads as a fundraising and list building tool more so than for voter persuasion during the primary season once he locks up the Democratic nomination."

Biden has emerged as the front-runner following victories in South Carolina on Feb. 29, Super Tuesday wins on March 3 and victories on March 10. His surge has been accompanied by a slate of endorsements from major figures in the Democratic Party and a massive boost in fundraising.

Forty-two percent of Sanders's Facebook ad budget was focused on states like Ohio and Florida from March 5-11. In Arizona, Sanders spent $104,175, quadrupling Biden's spending during the same period. Biden is leading Sanders by double digits in the state, according to a poll released Monday

Arizona, Florida and Illinois are still planning to hold their primary elections despite the coronavirus outbreak, which the World Health Organization designated a pandemic last week.

The disease has upended the Democratic primary, prompting Sanders and Biden to cancel in-person rallies and CNN to hold its latest debate without an audience.