A week later, the steady trickle of further gains in late results appear to reveal a far more positive picture for the party,
The Democrats have now picked up at least 32 seats in the House and are on course for four more, in addition to flipping seven governorships and eight state legislative chambers.
Though the Republicans are on track to increase their Senate by two seats, they had expected more.
On Monday night, Democrat Kyrsten Sinema won Arizona’s Senate race, beating Martha McSally in a landmark victory to take the seat held by retiring Republican senator Jeff Flake.
It means the overall results in the first nationwide election of Donald Trump’s presidency represent the Democrats’ best midterm performance since 1974, a vote which came in the wake of Watergate and Richard Nixon’s resignation.
Mr Trump, who last week declared election night to have been a “tremendous success”, has since been reduced to making baseless allegations of voter fraud to explain knife-edge contests in key seats such as Florida.
After securing a significant majority in the House, the Democrats have broken the Republican monopoly in Washington and secured the power to block the president’s legislative agenda.
“Over the last week we’ve moved from relief at winning the House to rejoicing at a genuine wave of diverse, progressive and inspiring Democrats winning office,” said Ben Wikler, Washington director of the liberal group MoveOn.
Conservative pollster Josh Jordan said there was a “striking” difference in the Democrat position on election night compared to a week later.
“They continue to pick up many of the close State, House, and even Senate seats in the days after,” he tweeted. ”What looked like a good night will definitely be thought of as a wave.”
Matthew Dowd, chief political analyst for ABC News, agreed. He pointed out the Democrats were on course to pick up more seats than in 2006 when Barack Obama was grappling with “a much worse economy”.
“That seems like a bit of a wave election to me,” he said.
Mr Trump and his allies have dismissed Democratic gains, pointing to GOP wins in Republican-leaning Senate states. Only once in the past three decades had a sitting president added Senate seats in his first midterm election.
However, the Democrats had been were defending 10 seats in states that Mr Trump carried just two years ago.
“The fact of the matter is, the Democratic Party had a historic night at the ballot box — and we are not resting,” said Democratic National Committee chairman Tom Perez. “Our goal was to compete everywhere, to expand and reshape the electorate everywhere — and that’s exactly what we’ve done.”
The Democrats found success by attracting support from women, minorities and university-educated voters. Overall, 50 per cent of white graduate voters and 56 per cent of women backed Democrats nationwide, according to Associated Press’s VoteCast poll.
Democrats featured historic diversity on their ballot. Their winning candidates included Massachusetts’ first African-American female member of congress, Ayanna Presley, as well as Michigan’s Rashida Talib and Minnesota’s Ilhan Omar, the first two Muslim women to serve in congress. Sharice Davids, elected in Kansas, became the first Native American woman and first lesbian Native American in congress.
“We have elected a new generation of inspiring leaders and we know that a new era of democratic dominance is on the horizon,” said Democratic Legislative Campaign Committee’s executive director Jessica Post.
The Democrats needed to gain 23 seats to seize the House majority. Once all the votes are counted, which could take weeks in some cases as absentees and provisional ballots are tallied, they could win close to 40.
They have not lost a single House incumbent so far, and defeated Republican targets such as Mike Coffman of Colorado, Barbara Comstock of Virginia, Carlos Curbelo of Florida, and Dana Rohrabacher of California.
They could win as many as 19 House races in districts carried by Mr Trump two years ago, according to House Democrats’ campaign team.