HICAGO, Feb. 20 — Amid the crowded field of Republicans vying for a seat in the Senate here, Jim Oberweis seems a most unlikely insurgent. He is a wealthy supporter of President Bush who favors pinstriped suits, tax cuts and a constitutional amendment blocking same-sex marriage.But in recent weeks, Mr. Oberweis, a plainspoken dairy owner, has become a leader in a widening conservative revolt against the president's sweeping plan to grant temporary legal status to millions of illegal immigrants."The president's plan is just plain wrong," Mr. Oberweis says in a radio advertisement and at public appearances that have drawn hundreds...