David Plouffe, the Democratic strategist who managed Barack Obama’s narrow 2008 presidential victory in North Carolina, is urging party donors and supporters to stay engaged in former Gov. Roy Cooper’s Senate campaign as Election Day approaches. On a call with supporters, Plouffe praised the campaign but warned that strong performance so far is no reason to assume victory. “I think that this race ... has I think been the best run race in the country,” he said. “But that doesn’t mean that we can rest easy.” His warning comes as national Democrats begin redirecting some attention from North Carolina. Polls show Cooper ahead, and party strategists are looking at other contests once considered long shots as potential opportunities to gain seats. Plouffe’s comments underscore the risk of pulling back resources before votes are cast. North Carolina remains a closely divided, or “purple,” state, Plouffe said, limiting how much either campaign can expect to expand its support. That balance makes turnout and sustained campaigning especially important in the final weeks. Cooper, a former governor, is seeking a Senate seat in a state where the outcome could help shape control of the chamber. Plouffe’s appeal reflects a familiar challenge for Democrats: deciding how to distribute campaign money and attention without treating a lead in polling as a guaranteed win.