For the last five months, State Senator Thomas H. Kean Jr., the son of a revered former governor, has trained all of his political fire at the feisty Democratic power broker he wants to unseat, United States Senator Robert Menendez. But now, in a twist, it appears that Mr. Kean will have to contend first with a political distraction that he worked for more than a year to avoid: a challenger in the Republican primary. The potential challenger's name is John P. Ginty, and he is a 41-year-old financial data analyst and former president of the Ridgewood Republican Club who...