From a political standpoint, it should have been an easy decision. The calls flooding Fred D. Thompson’s Senate office in the winter of 1999 showed that his Tennessee constituents overwhelmingly favored removing President Bill Clinton from office. But as the historic impeachment trial neared, records show, Mr. Thompson agonized over what he saw as two “bad choices...” But his approach to the impeachment case — and his ultimate decision to part with the Republican majority by voting to acquit Mr. Clinton on one of two impeachment counts — underscores the concerns now being raised by many conservative leaders.Less than a...