U.S. Rep. Greg Walden (R-Ore.) was a lock next year to retain the 2nd Congressional District seat he's held since 1999. Republicans outnumber Democrats by 55,000 registered voters or 8 percent of the electorate in the mostly rural district that sprawls across most of Eastern and Southern Oregon.

Last year, when President Donald J. Trump's toxicity weighed heavily on the Oregon electorate, Walden, the only Republican member of Oregon's congressional delegation, defeated Democratic challenger Jamie McLeod-Skinner by 17 percentage points, which means the district is even redder than its voter registration.

And Walden is one of the most senior members of his caucus, able to name his assignment if the GOP ever takes back the House.

Yet on Monday, Walden called it quits.

"It was a surprise," says Pacific University political science professor Jim Moore. "In the past three months, Walden's been more open about saying he's got problems with what the president is doing, especially the Ukraine stuff. I think that's a big part of it."

Trouble for President Donald J. Trump deepened Oct. 28, when a U.S. Army officer assigned to the White House released prepared testimony saying he was disturbed by Trump's demand that Ukrainian officials investigate his political opponent Joe Biden.