Massad would take over the task of implementing the remaining rules. For many, a key question is whether he will exercise independence from the administration and the banks, as Gensler often did.
Gensler, who had worked for nearly 20 years on Wall Street, surprised many by being a tough regulator of banks. He pushed for stricter rules that banks had lobbied against. And he wasn't afraid to take positions that clashed with the Obama administration.
Massad has worked for the Treasury since Obama took office in 2009 and has been an advocate for the administration's policies.
"The question is whether he has the guts, independence and commitment ... to stand up to Wall Street," said Dennis Kelleher, the president of Better Markets, a group that advocates strict financial regulation. "It's a dramatically difficult job at an independent agency at a critical time."
For the CFTC, some of the thorniest and most critical rules are among those that remain. They include the so-called Volcker Rule, which would prohibit banks from trading for their own profit. Its latest version includes an exemption for banks to make such trades when they are used to offset other risks taken. Adoption of the rule has been delayed largely because of Wall Street banks' objections and the need to get a handful of federal agencies, including the CFTC, to agree on its final form.
Under Gensler, the CFTC wrote new rules to bring derivatives under government supervision for the first time. Derivatives were blamed as the accelerant on the fire that ignited the financial crisis.
The value of derivatives is based on a commodity or security, such as oil, interest rates or currencies. They are often used to protect businesses that produce or use the commodities, such as farmers or airlines, against future price fluctuations. But they also are used by financial firms to make speculative bets.