Republican strategist Karl Rove says Sen. Bernie Sanders Bernie SandersOutrage erupts over Breonna Taylor grand jury ruling Dimon: Wealth tax 'almost impossible to do' Grand jury charges no officers in Breonna Taylor death MORE (I-Vt.) could defeat President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE in 2020.

In an opinion piece in The Wall Street Journal Thursday, Rove cited Sanders's performance at a Monday town hall on Fox News, during which the audience applauded his calls for "Medicare for All."

“When only 37% of Americans in the RealClearPolitics average think the country is going in the right direction while 56.4% think it’s on the wrong track, Mr. Sanders could be perceived as an agent of change,” Rove wrote.

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“If he is the Democratic nominee, Mr. Trump’s task will be to convince Americans that a socialist turn would be a ruinous change. Based on Monday’s town hall, that won’t be as easy as Republicans may think. Mr. Sanders is a real contender,” he added.

Rove wrote that Sanders showed he has improved as a candidate since 2016, citing how he sidestepped a question about whether he benefitted from the Republican tax cuts to focus instead on Trump's refusal to release his own returns.

Rove added that Sanders had learned how to “smooth socialism’s rough edges" and now had a message focused on economic disparities that resonated with voters.

Polls show Sanders as one of the top-tier Democratic presidential candidates, running second behind former Vice President Joe Biden Joe BidenBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Democratic groups using Bloomberg money to launch M in Spanish language ads in Florida Harris faces pivotal moment with Supreme Court battle MORE in most polls.

A national poll last week showed him in first place and as the top beneficiary if Biden, who has still not formally announced, decides not to run.

Sanders’s Fox News town hall drew more than 2.55 million viewers as well as the ire of Trump, a frequent Fox News viewer, who accused the network of not allowing Trump supporters into the event.