SALEM, Ore. — The lone Republican in Oregon’s congressional delegation won’t seek reelection to a 12th term, throwing a huge district covering a conservative part of the state up for grabs. With less than seven months to go before the 2020 primaries, Rep. Greg Walden’s videotaped announcement on Monday sets up further changes in the U.S. House of Representatives, which Democrats regained control of in the 2018 midterm elections. Walden is the 19th House Republican to announce he or she will not seek re-election. Three other GOP lawmakers have resigned. Walden is a former chairman of the powerful House Energy and Commerce Committee and now serves as the top Republican on the energy panel. He was a key player in GOP efforts to replace President Barack Obama’s health care law but was considered a moderate. The surprise resignation injected drama into Oregon politics, with the race for secretary of state having been considered the most compelling. One of the Democratic cand