WASHINGTON—Wall Street’s watchdog on Wednesday unveiled a draft new rule to enhance blank-check company investor disclosures and to strip them of a legal protection critics argue has allowed the shell companies to issue overly optimistic earnings projections. The move by the U.S. Securities and Exchange Commission (SEC) is part of a broader crackdown on special purpose acquisition companies (SPACs) after a frenzy of deals in 2020 and early 2021 sparked concerns that some investors are getting a raw deal. Wall Street’s biggest gold rush of recent years, SPACs are shell companies that raise funds through a listing to acquire a private company and take it public, allowing the target to sidestep the stiffer regulatory scrutiny of a traditional initial public offering (IPO). The SEC proposal, which is subject to consultation, broadly aims to close that loophole by offering SPAC investors protections similar to those they would receive during the IPO …