WASHINGTON - Breaking with party leaders, a Democratic congressman plans to introduce Social Security legislation, saying his first commitment is to his constituents. Rep. Robert Wexler, D-Fla., said Friday: "I have the largest amount of Social Security recipients of any Democrat anywhere in the country. My allegiance to seniors is greater than my allegiance to the Democratic Party." While Wexler is proposing tax increases that would clash with President Bush's pledge not to expand the existing payroll tax, his legislation was heralded by the White House in part because Democrats had steadfastly refused to offer an alternative to Bush's plan....