* Memo builds on executive order issued in January

* TransCanada has yet to apply to build southern line

WASHINGTON, March 21 (Reuters) - President Barack Obama will send a memo to federal agencies on Thursday directing them to prioritize permitting for TransCanada's southern leg of the Keystone oil pipeline, a senior White House official said on Wednesday.

Facing a barrage of Republican criticism over high gasoline prices during the election year, Obama will visit Cushing, Oklahoma, on Thursday to promote his energy policies, which include support for the southern leg of the pipeline.

The pipeline would drain a glut of crude in Cushing, the storage hub for U.S. crude oil traded on the futures market, easing deliveries to refineries along the Gulf Coast.