Elon Musk‘s latest target on Twitter is U.S. Sen. Bernie Sanders, a longtime advocate for taxing billionaires and redistributing wealth.

Bernie Sanders tweeted, “We must demand that the extremely wealthy pay their fair share. Period.” on Saturday afternoon. The tweet was directed towards his followers, not Musk in particular.

One day later, Musk responded to the 80-yearTwitter thread saying, “I keep forgetting that you’re still alive” in a tweet.

He followed with another taunting tweet an hour later, saying, “Want me to sell more stock, Bernie? Just say the word …”

Musk recently sold $1.1 billion in Tesla stock after polling his followers on Twitter about whether or not he should sell 10% of his shares. In a poll tweet that garnered over 3.5 million votes, 57.9% of respondents said yes and 42.1% said no.

Tesla became the most valuable American carmaker in 2020, and its current valuation is over $1 trillion, eclipsing veteran car makers such as Ford and General Motors.

Musk has now become the wealthiest person in the world and is set to become the first trillionaire in history.

A ProPublica investigation released in June showed that many of the world’s richest billionaires don’t pay a cent in income taxes. They found from IRS records that in 2018, Musk, the second-richest person in the world at the time, paid no federal income taxes.

IRS records obtained by ProPublica also showed that Jeff Bezos, then a multibillionaire and now the world’s second richest man, did not pay a penny in federal income taxes in 2007 and 2011.

Michael Bloomberg managed to do the same in recent years. Billionaire investor Carl Icahn did it twice. George Soros paid no federal income tax three years in a row.

Sanders and U.S. Sen. Elizabeth Warren proposed in March a 2% annual tax on wealth over $50 million, rising to 3% for wealth over $1 billion, according to CNBC. Sanders and Warren have been advocating for a wealth tax for years and both made it a central part of their platforms.

During the 2020 presidential election cycle, then-Vice President Joe Biden criticized the Warren and Sanders plans as “demonizing wealth” and argued instead for focusing on income taxes and raising the rates charged on earnings from investments.