WASHINGTON—President Joe Biden said on Friday he planned to nominate Mallory Stewart to be assistant secretary of state for arms control, a key job as Washington and Moscow contemplate a successor to the New START arms control treaty. If confirmed by the Senate, Stewart, now a senior director on the White House National Security Council, would lead the State Department’s bureau of arms control, verification and compliance, the White House said in a statement. The United States and Russia in February extended New START for five years. The treaty, which first went into effect in 2011, limits the United States and Russia to deploying no more than 1,550 strategic nuclear warheads each and imposes restrictions on the land- and submarine-based missiles and bombers to deliver them. Historically, the assistant secretary of state for arms control plays a central role in negotiating such accords. At their June 16 Geneva summit, Biden …