In a Senate race that could have implications for the 2008 presidential contest, Sen. Hillary Rodham Clinton raised more than $6 million between April and June. Campaign reports filed by Senate candidates Friday with the Federal Election Commission showed the former first lady, a New York Democrat, had $12.6 million cash on hand at the end of last month, even though she has no clear opponent yet. Manhattan lawyer Edward Cox, a son-in-law of former President Nixon, has begun preparations to challenge Clinton and Republican party officials also are courting Westchester County District Attorney Jeanine Pirro as a possible challenger....