With Republican Senator Ted Cruz, the leader in Iowa polls, spending the week effectively off the campaign trail, his rivals seemed to sense an opening: zigzag the state, press the flesh and make an early impression before the field descends on Iowa after New Year’s. On Monday, though, some candidates’ best plans were upended by an opponent that could outpace even Donald J. Trump in bluster: the weather. With several inches of snow, and icy conditions and howling winds expected in some parts of the state, Senator Marco Rubio of Florida and Gov. Chris Christie of New Jersey were forced...