In Washington State, Democrats won a special election to take control of the State Senate, establishing total Democratic dominance of government on the West Coast. Democrats took council seats in vote-rich Delaware County, in the Philadelphia suburbs, a perennial battleground for control of the House.
Even in the Deep South, Georgia Democrats captured two state House seats where they previously had not even fielded candidates while snatching a State Senate seat in Buckhead, Atlanta’s toniest enclave.
“Republicans are being obliterated in the suburbs,” said Chris Vance, a former chairman of the Washington State Republican Party. “I don’t think the Republican Party has a future in any state like Washington or Virginia, or Oregon or California, or many other places, where the majority of the voters are from urban or suburban areas.”
Mr. Vance placed the blame squarely on Mr. Trump: “Among college-educated suburbanites, he is a pariah.” In Washington, D.C., congressional Republicans braced for a new wave of retirements just one day after another pair of House members, veteran Representative Frank LoBiondo of New Jersey and Representative Ted Poe of Texas, declared they would not seek re-election. Mr. Dent, channeling the exasperation of his colleagues, suggested an exodus might be imminent.
“Our guys know they’re going to be running into a fierce storm,” said Mr. Dent, a leader of his caucus’s moderate wing who has already announced he will not run again. “Do they really want to go through another year of this?”
Even in the White House, where Mr. Trump’s first reaction was to savage Ed Gillespie, the party’s defeated gubernatorial candidate in Virginia, two advisers acknowledged on Wednesday morning that Mr. Trump was likely to help drive Democratic turnout next year in much the same way his predecessor, Barack Obama, did for conservative voters during midterm elections.
Democrats were as buoyant as Republicans were dejected. Party leaders gleefully predicted that the Senate, where the Republicans hold a two-seat majority, might now be in play, and they said that their fund-raising and candidate recruitment would take off going into the new year.
“We’ll get a lot of candidates who are going to want to run, and I think for donors who have been on the sidelines, dispirited for the last year, I’m telling you people are jazzed up,” said Gov. Terry McAuliffe of Virginia, the ever-upbeat former national Democratic Party chair.
Credit Gabriella Demczuk for The New York Times
Democrats still face formidable obstacles in the 2018 election, including some not at work in this week’s elections. If a suburban insurrection might help Democrats take the House, the Senate seats at stake next year are overwhelmingly in conservative, rural states, where feelings about Mr. Trump range from ambivalent to positive. So far, only two Republican Senate seats appear in play, the Arizona seat being vacated by Jeff Flake and Dean Heller’s seat in Nevada.
In House races, Democratic candidates are likely to face Republican attacks tying them to Representative Nancy Pelosi, the unpopular Democratic minority leader, and a range of liberal policies, like single-payer health care, that are causing divisions in the Democratic ranks.
But to many Democrats and, much to their consternation, some Republicans, the results recalled the last time a radioactive Republican was in the White House and voters took out their frustrations on a Republican-held Congress. In 2005, Democrats rolled to victory in Virginia and New Jersey, presaging a wave election in 2006, and inspiring throngs of Democrats to run for office in difficult districts.
Representative Ben Ray Luján of New Mexico, chairman of the Democratic Congressional Campaign Committee, said he had spent Tuesday evening calling potential House candidates and urging them to watch the returns, joking: “I just want to encourage you to turn on the television, if it’s not already on.”
Mr. Luján said the results would embolden Democrats to contest an ambitious list of races in 2018. The party has already been pursuing more than a dozen seats across the states that voted on Tuesday night, including some that overlap heavily with areas in Virginia and New Jersey where Democrats won by landslide margins.
“Democrats down there were very aggressive about expanding their map and recruiting strong candidates, even where they were told they couldn’t win,” Mr. Luján said of Virginia. “We’re going to make our Republican colleagues fight for every inch.”
At the Senate level, too, Democrats are seeking to expand the map. Mindful of their narrow path to a majority, they are strenuously wooing Phil Bredesen, a former Tennessee governor, to run for the seat that Senator Bob Corker is vacating. Mr. Bredesen has been courted personally by Senator Chuck Schumer of New York, the minority leader, as well as several former governors who now serve in the Senate, including Mark Warner of Virginia, according to Democrats briefed on the overtures. And the Democratic Senatorial Campaign Committee commissioned a poll aimed at coaxing him into the race.
Mr. Bredesen is in Washington this week for meetings and is said to be nearing a decision.
Democrats won on Tuesday with a historically diverse slate of candidates: Having long struggled to bring diversity to the leadership tier of their party, they elected the first transgender legislator in the country, the first Vietnamese-American legislator in Virginia, the first African-American female mayor of Charlotte, N.C., and the first black statewide officer in Virginia in more than a quarter-century, among other groundbreaking candidates.
Kathy Tran, who was elected to the House of Delegates in a Fairfax-based seat that Republicans previously held, said voters in her district had mobilized to rebuke Mr. Trump and his brand of politics. She urged national Democrats to follow Virginia’s example by recruiting candidates from a range of backgrounds for the midterm campaign.
“This was a clear rejection of racism and bigotry and hateful violence,” Ms. Tran said of the elections, adding: “People are hungry for a government that reflects the diversity of our communities.”
County-level results captured the dizzying scale of the lurch away from Republicans: In Virginia, Lt. Gov. Ralph S. Northam captured the outer Washington suburbs of Prince William and Loudoun County by 20 percentage points or more. Four years earlier, Governor McAuliffe, a fellow Democrat, won both areas by single digits. In the traditional Republican stronghold of Chesterfield County, outside Richmond, Mr. Northam trailed Republican Mr. Gillespie, by less than 300 votes. And in Virginia Beach, which Mr. Trump even carried while losing the state, Mr. Northam won by 5 percentage points.
In New Jersey, Mr. Murphy carried the densely populated New York and Philadelphia suburbs by staggering margins. He won Middlesex County, a politically influential suburb southwest of New York City, and Bergen County, the state’s most populous locality, by about 15 percentage points each. Eight years earlier, Gov. Chris Christie, a Republican, carried Middlesex and nearly matched his Democratic opponent in Bergen, strong showings that made his narrow statewide victory possible.
And in Delaware County, Pa., long home to a fearsome Republican machine, Democrats won seats on the county council for the first time since the 1970s thanks to a local campaign that featured yard signs that got straight to the point: “Vote Nov. 7th Against Trump.”
Robert F. McDonnell, the former governor of Virginia, and the last Republican to win a major election in the state, acknowledged on election night that the electorate there had soured on his party. The state, he said, had been swamped by “anger and malaise and vitriol” emanating from federal politics, and Democrats benefited from the electric energy of their base.
“The enthusiastic left showed up tonight in big numbers,” Mr. McDonnell said, “and that really determined the outcome.”