Sen. Ron Wyden’s proposal to tax the unrealized gains of billionaires would harm capital markets, lowering the number of public companies and reducing the number of people who participate in stock markets, critics say.

Wyden, a Democrat from Oregon, recently released a mark-to-market capital gains tax proposal after years of planning. The billionaire tax was floated as a way to pay for the Democratic climate and social spending plan, although it quickly was placed on the back burner after Democratic Sen. Joe Manchin of West Virginia expressed opposition.