New Hampshire Gov. Chris Sununu on Tuesday endorsed former South Carolina Gov. Nikki Haley, a potential boost to her bid in the state that will hold the second Republican presidential nominating contest on Jan. 23.

The move by Sununu, a popular governor who is strongly opposed to former President Donald Trump’s effort to win the GOP nomination for a third time, is a blow to the New Hampshire-focused campaign of former New Jersey Gov. Chris Christie.

Sununu formally announced his support for Haley, also a former United Nations ambassador, at an evening event in Manchester.