Already thinking about his 2020 re-election bid, President Donald Trump essentially dared Democratic voters Thursday evening to pick a nominee from that party’s most liberal ranks.

Trump often lets the world know his thoughts on legislation, policy decisions and foreign policy matters — either during free-wheeling political rallies or Twitter rants. That was the case during a campaign rally in Wilkes-Barre, Pennsylvania, when he signaled he would prefer to run against “Pocahontas” — his racially tinged nickname for Massachusetts Democratic Sen. Elizabeth Warren — or “Crazy Bernie” — his dismissive moniker for Vermont independent Sen. Bernie Sanders.

Both are leaders of the Democratic Party’s progressive wing. And both are eyeing 2020 presidential bids. But the president seemed to use his signature nicknames for his political foes Thursday night for a larger purpose than merely getting a reaction from an arena full of “Make America Again” gear-wearing supporters.

[So… How Does the White House Really Feel About Russia?]

He sketched out how he would counter messages from a liberal Democratic nominee — and notably, he did not mention more centrist Democrats who are considering a presidential bid, such as former Vice President Joe Biden, who already has a 7-percentage point lead over Trump in a hypothetical one-on-one race, according to a new poll.