While President Donald Trump has been in office for less than a year, he is already eyeing his potential opponents in 2020. At the top of the list for possible nominees to challenge the incumbent is Facebook CEO Mark Zuckerberg, Politico reported.

The field for the top spot on the Democratic party’s ticket in 2020 is considered wide open, with the social network founder at the top of list. Sen. Elizabeth Warren, D-Mass., and a number of other veteran office holders and outsider candidates are also being watched by the Trump administration.

Zuckerberg has fueled the speculation of a potential presidential run in recent years. Once relatively shy of speaking out, the Facebook CEO has taken to using his platform to express his opinions and champion causes he cares about.

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During the 2016 presidential election, Gizmodo obtained a screenshot of an internal Facebook employee discussion in which Zuckerberg was asked, "What responsibility does Facebook have to prevent President Trump in 2017?" Trump, responding on Fox & Friends, expressed support for Zuckerberg and labeled himself a "star" on Facebook's social media platform.

He also stirred up conversation about his political aspirations when he announced he would travel to all 50 states in the United States in 2017. He has posted photos from communities he has visited and written about his experiences meeting local leaders and talking to citizens of the country who live outside the Silicon Valley bubble.

Zuckerberg said the trip across the country, which he calls a listening tour, was designed so he could talk about how “people are living, working and thinking about the future.”

Earlier this year, the Chan Zuckerberg Initiative— a foundation created by Zuckerberg and his wife Priscilla Chan— hired Benenson Strategy Group, an advisory firm run by former Democratic pollster Joel Benenson. Benenson worked both with the administration of Barack Obama and the Hillary Clinton campaign in 2016.

The Trump administration hasn’t engaged much directly with Zuckerberg or his apparent political ambitions, though it has be subtly taking adversarial positions against the social network founder and his company.

Prior to being let go from his role as White House chief strategist, Steve Bannon began floating the possibility of looking at Facebook as a monopoly and regulating it as a public utility, giving the federal government more control over how the service operates. According to Politico, that policy was made public as a way to “troll” Zuckerberg.

Zuckerberg has often tiptoed around directly attacking the Trump administration. While the tech mogul has been critical of the president’s policies, he has often shied away from a full-throated critique.

The harshest criticism he has offered came after President Trump signed an executive order that would ban immigrants and refugees from a number of majority-Muslim countries. Zuckerberg took to Facebook to express dismay at the policy.

“Like many of you, I’m concerned about the impact of the recent executive orders signed by President Trump,” he wrote at the time. “We need to keep this country safe, but we should do that by focusing on people who actually pose a threat.”

Facebook joined a coalition of more than 160 tech companies that signed onto a filing asking a federal court to reject the travel ban proposal as unconstitutional.

In addition to Zuckerberg, Trump and the Republican National Committee have taken aim at Democratic lawmakers including Sens. Sherrod Brown of Ohio, Cory Booker of New Jersey, Kirsten Gillibrand of New York, Kamala Harris of California, Amy Klobuchar of Minnesota and Rep. Seth Moulton of Massachusetts. Starbucks CEO Howard Schultz is also viewed as a potential outsider candidate who may compete for the nomination.