WASHINGTON (Reuters) - U.S. President Donald Trump is expected to nominate Columbia University law professor Robert Jackson to be a member of the Securities and Exchange Commission, a person with knowledge of the matter said on Thursday. Jackson is expected to be formally nominated on Friday, the source said. He would fill a vacant spot reserved for a Democrat on the five-member panel. His appointment follows the administration’s decision to nominate Hester Peirce as a Republican commissioner in July and would bring the SEC to full strength and paving the way for Chairman Jay Clayton to push ahead on an agenda of reducing public companies’ regulatory burdens. Jackson could not be reached for comment on Thursday. Jackson is known for his advocacy work in trying to advance new rules at the SEC that would force public companies to disclose their political spending to investors. 