WASHINGTON (Reuters) - A bipartisan group of dozens of former U.S. Homeland Security  officials sent letters supporting the nomination of Kirstjen Nielsen as the new head of the department ahead of her confirmation hearing on Wednesday by the Senate Homeland Security Committee.  Among the signers were former Homeland Security (DHS) secretaries Tom Ridge and Michael Chertoff, both Republicans who served under former President George W. Bush.  “Ms. Nielson has been engaged in counter-terrorism, all-hazard risk mitigation, critical infrastructure protection, and response policy from the earliest days of what we now know as homeland security,” the two former secretaries wrote in a latter viewed by Reuters.  A separate letter was signed by former National Security Agency Director Keith Alexander and former DHS officials Charles E. Allen, James M. Loy and John McLaughlin. The letter was also signed by Jane Holl Lute and Alejandro Mayorkas, both who served as under secretaries of DHS under former President Barack Obama. If confirmed by the Senate, Nielsen would take the reins at a sprawling department with more than 240,000 employees that is responsible for U.S. border and airport security, immigration policy, disaster response, refugee admissions and other matters. Nielsen, 45, is a cyber security expert with a considerable resume in homeland security work that includes service at the department’s Transportation Security Administration and on former Republican President George W. Bush’s White House Homeland Security Council. She was nominated to the role after John Kelly, the previous secretary, was named White House chief of staff this summer. Nielson worked as Kelly’s deputy in the White House prior to being nominated in October. 