WASHINGTON (CNN) -- On the eve of a showdown over a historic overhaul of U.S. immigration law, congressmen drew lines in the sand Sunday, leaving it all but impossible to envision what kind of legislation may ultimately win passage. "It is incumbent upon us in the Senate to compromise," Senate Majority Leader Bill Frist told CNN's "Late Edition." He called the nation's immigration system "flat out broken." The fierce debates -- centering mostly on what to do with the millions of undocumented immigrants currently in the country -- don't fall along a simple partisan divide. Splits within each party and...