When the presidential campaigns bombarded Iowa over the summer, there was an unintended consequence: Every House race in the state became more competitive. Consequently, a state once expected to feature one or two competitive House races could see all four seats in play. This newfound dynamic slightly favors Republicans, who face tough climbs to unseat Democratic incumbents in two eastern Iowa seats. But Democrats also reap the benefits of a more competitive field that is energizing their candidates in the western half of the state, including Rep. Leonard Boswell (D) in his marquee battle with Rep. Tom Latham (R). The...