George Voinovich's opposition last year to President George W Bush's choice helped derail the confirmation vote, though Bush sidestepped the obstacle by appointing Bolton during a Senate recess.
Bolton's tenure ends when the Senate goes into recess this fall and Voinovich's shift could help build support for a renewed nomination.
Voinovich now says he'll support Bolton if Bush proposes him again because the Security Council is facing crucial decisions on North Korea, Iran and the Middle East and the world must know that Bolton speaks for the United States.
'I do not believe the United States, at this dangerous time, can afford to have a UN ambassador who does not have Congress's full support,' Voinovich wrote in Thursday edition of the Washington Post.
Democrats opposed Bolton because of allegations that he berated colleagues who disagreed with his views, worked behind the back of higher-ranking officials and held the UN in disdain.
Bush nominated Bolton because of his strong views that the world body needed reform.
'I believe Bolton has been tempered and focused on speaking for the administration,' Voinovich said.
