At a news conference outside the Capitol, Lewis cited Marshall's passion for addressing the inequalities in American life, and that she was not the candidate chosen by the Democratic Senatorial Campaign Committee, reports Rob Christensen.
"She did not back down from Washington insiders trying to exercise undue influence in our nominating process," Lewis said. "Instead, Secretary Marshall has shown the kind of courage, strength and independence needed to create change in Washington and fight for the people of North Carolina."
Both Marshall, the secretary of state, and Cal Cunningham, the former state senator from Lexington, had avidly courted Lewis, a Chapel Hill lawyer. The runoff is June 22.
Marshall led the first primary with 36 percent, followed by Cunningham with 27 percent, and Lewis with 16 percent, with the rest of the vote spread among three other Democratic candidates.
As the leading African-American candidate, Lewis' endorsement has been given extra importance. But an endorsement does not necessarily mean that a candidate's supporters will follow.
Lewis encouraged his backers to work for Marshall and said he planned to campaign for her. At least three of his campaign aides plan to join the Marshall campaign.
Lewis waved off criticism he made during the primary, in which he indirectly criticized Marshall as a career politician, saying he had gotten to know Marshall better during subsequent post-primary conversations.