President Donald Trump welcomed Sen. Bernie Sanders (I-Vt.) into the presidential race on Wednesday by reviving an old nickname — but the Vermont senator isn’t having it.

“Crazy Bernie has just entered the race,” Trump wrote in a Wednesday morning tweet. “I wish him well!”

Crazy Bernie has just entered the race. I wish him well!

— Donald J. Trump (@realDonaldTrump) February 20, 2019

Trump’s backhanded tweet about Sanders, who unsuccessfully faced off against former Secretary of State Hillary Clinton for the Democratic Party’s nomination in 2016, didn’t sit well with the Vermont senator.

In a subsequent tweet, Sanders challenged Trump directly, giving his own view of what is “crazy.”

“What’s crazy is that we have a president who is a racist, a sexist, a xenophobe and a fraud,” he wrote. “We are going to bring people together and not only defeat Trump but transform the economic and political life of this country.”

What’s crazy is that we have a president who is a racist, a sexist, a xenophobe and a fraud. We are going to bring people together and not only defeat Trump but transform the economic and political life of this country. Say you're in: https://t.co/rxqbjJJKv3 https://t.co/HoJG0Rlwq1

— Bernie Sanders (@BernieSanders) February 20, 2019

Sanders also turned Trump’s tweet into a fundraising opportunity, including a link in the tweet that prompts users to sign up for the campaign’s email list and donate $3, $27, or a custom amount.

The viral response to Trump’s tweet is just the latest step in a whirlwind fundraising spree for the first 48 hours of the Sanders’ latest bid for the presidency. The campaign reported over $6 million in donations, largely from small donors, in just one day.

Trump’s use of the “Crazy Bernie” nickname dates back to the 2016 Democratic primary when Trump would seemingly encourage Sanders as a more subtle way of attacking the Democratic front-runner, Clinton.

Big wins in West Virginia and Nebraska. Get ready for November – Crooked Hillary, who is looking very bad against Crazy Bernie, will lose!

— Donald J. Trump (@realDonaldTrump) May 11, 2016

I don't want to hit Crazy Bernie Sanders too hard yet because I love watching what he is doing to Crooked Hillary. His time will come!

— Donald J. Trump (@realDonaldTrump) May 11, 2016

Trump would later revive the nickname during his presidency as a means of arguing that Clinton and the Democratic Party treated Sanders unfairly in the primary process:

Hillary Clinton colluded with the Democratic Party in order to beat Crazy Bernie Sanders. Is she allowed to so collude? Unfair to Bernie!

— Donald J. Trump (@realDonaldTrump) June 25, 2017

The president commented on Sanders’ candidacy from the Oval Office on Tuesday, expressing a belief that the senator had “missed his time” to run for the presidency.

Trump said Sanders’ candidacy would be “interesting” to watch but then pivoted to criticizing the 2016 Democratic primary process.

“I think what happened to Bernie, maybe, was not so nice,” Trump said.

Watch the video below, via CNN:

Trump reacts to Sen. Bernie Sanders announcing his second presidential campaign: "Personally, I think he missed his time. … But, I wish Bernie well, it'll be interesting to see how he does" https://t.co/ICdEBTk3Im pic.twitter.com/YJqUie6105

— CNN Politics (@CNNPolitics) February 19, 2019

“I think he was taken advantage of,” Trump added. “He ran great four years ago, and he was not treated with respect by Clinton.”