CONCORD, N.H. — Donald Trump returns to the Republican presidential debate stage here Saturday night facing a crucible. Marco Rubio arrives as a sudden star, basking in the establishment's embrace but fending off ferocious attacks from his rivals. Still glowing from his Iowa victory, Ted Cruz is laboring to further consolidate conservative support, while Ben Carson is struggling to avoid being typecast as the dead man walking — or, in this case, talking.