PARIS (Reuters) - Hundreds of thousands of demonstrators joined rallies across France on Tuesday in a fresh assault by students and striking public sector workers on a youth hire-and-fire contract. Two months of sometimes violent demonstrations took their toll on the popularity of the contract's main champion, Prime Minister Dominique de Villepin, whose ratings in a new poll slid 14 points in a month to 28 percent in March. Unions said the turnout should reach the three million figure seen in last week's protests, among the biggest in France's 48-year-old Fifth Republic. Police estimates were not immediately available but were...