Wall Street is fighting back as Securities and Exchange Commission Chairman Gary Gensler considers policy changes that threaten to upend a lucrative business model.

Trading firms and brokers have ramped up their lobbying efforts and campaign donations to Republicans seeking to win control of Congress in next year’s midterm elections. They’ve become especially vocal about Gensler’s scrutiny of payment for order flow, whereby some brokerages sell their clients’ stock and option orders to high-speed trading firms that execute the trades.