President Trump on Thursday tapped Sen. Markwayne Mullin (R-Okla.) to replace Kristi Noem as secretary of the Department of Homeland Security (DHS), creating a vacancy in the U.S. Senate and setting off a scramble to replace him.  Oklahoma Gov. Kevin Stitt (R) has 30 days to appoint someone from Mullin’s party to temporarily fill the seat after the senator leaves his post, but the appointee must pledge not…