Republican advances in traditionally Democratic states, including Connecticut, Oregon and Washington, may not translate into a wave of GOP victories. But they have rattled local campaigns and forced the Democrats to shift attention and money to races they didn’t expect to be defending.

Rising sentiment against the party in power has washed ashore even in coastal Oregon, where Democratic Rep. Peter DeFazio won his 10th re-election two years ago with 82% of the vote.

“I am having the same problem that Democrats are having across the country, which is ennui,” he said, noting that his opponent’s yard signs “are thick” across much of the district. Mr. DeFazio said he is facing the fight of his political life…

The expanded battlefield map, however, has prompted a shift in tone. Oregon Rep. Greg Walden, vice chairman of the National Republican Campaign Committee, bluntly predicted his party is heading toward a big win. “The Democrats are standing on a beach with the water going out and there is a tsunami coming their way,” he said.