"This does not seem like particularly new information," said Michael Reinking, senior market strategist at the New York Stock Exchange, in a note.

To be fair to the worried investors, the proposed rates look very different from the current ones, with a top federal capital gains tax of only 20%. Individuals earning $200,000 a year and married couples making $250,000 a year also pay an additional 3.8% tax on their capital gains to help fund Obamacare.

Wall Street is not impressed and the shock from these numbers is hard for investors to shake off, said Edward Moya, senior market analyst at Oanda, in a note to clients.

"The biggest risks to the stock market are the Fed's taper tantrum and aggressive tax hikes," he said.

That said, this is only a proposal and the final product might look very different. White House press secretary Jen Psaki said on Thursday that details of the American Families Plan will be outlined next week during President Joe Biden's address to Congress, but she would not confirm tax increase details in the plan.

Stocks remain near record highs, so these market wobbles could be a good excuse for investors to sell shares and bank some profits, Moya added.

--Tami Luhby and Katie Lobosco contributed to this report.