“He’s looking all but certain to do it,” said a former top aide to Sanford, who did not want to be identified while prematurely revealing the plans.

A formal announcement will come soon, the source said.

Sanford intends to seek the Republican nomination in South Carolina’s 1st Congressional District, a seat being vacated by Rep. Tim Scott, who was appointed to the United States Senate earlier this week by Gov. Nikki Haley.