The immigration and security debate, which has sparked huge demonstrations in recent days by Hispanic residents of cities around the country, comes at a crucial moment for both parties. Over the last three national elections, persistent appeals by Mr. Bush and other Republican leaders have helped double their party's share of the Hispanic vote, to more than 40 percent in 2004 from about 20 percent in 1996. As a result, Democrats can no longer rely on the country's 42 million Hispanic residents as a natural part of their base. In a lunch meeting of Senate Republicans this week, Senator Mel...