Washington: Donald Trump is reviving memories of Bill Clinton's affair with a White House intern and his turbulent interactions with black voters during South Carolina's 2008 primary as the ex-president campaigns for his wife in New Hampshire.

Trump's latest broadsides on the Clintons a potential preview of a nasty, personal general election appear beneficial to both as they seek to energise voters leading into the first primary contests. But, some observers warn, they could pose a long-term risk for Trump in his effort to succeed President Barack Obama.