By Nell Mackenzie (Reuters) – Senate Republicans want the SEC to explain why staff are leaving the nation’s corporate watchdog at the highest rate in 10 years amid a flurry of proposed rules, according to a letter seen by Reuters on Sunday. The private letter dated Oct. 27 from Senate Republicans to the chair of the Securities and Exchange Commission, Gary Gensler, adds to mounting criticism that the U.S. regulator lacks the internal firepower it needs to accomplish its ambitious rulemaking plans. Gensler, a veteran Wall Street regulator who was chosen by President Joe Biden, a Democrat, has a…