Obviously you don't follow politics a lot. Kerry did not endorse

Clinton because in 2004 Clintons drained a lot of resourses including funds into Hillary's New York Senate run and saved for his wife's 08 Presidential campaign. It indirectly derailed Kerry's campaign. Also it was not in Clinton's interest to see White House occupied by another non-Clinton in 2004. Otherwise, 2008 would have been President Kerry seeking re-election or 2012 been Democrats fighting to keep White House.