The move has polarised voters and fractured the long-standing political consensus.
Given this backdrop, Dr Wellings said he was actually surprised the Sweden Democrats did not capture even more of the vote.
"This vote forces us to rethink some of the assumptions we have made about European social democracies and really consider the vulnerability of these liberal and socially democratic models to populist critique."
Dr Wellings said the centre-left has "a real problem on their hands in articulating a vision of European society in ways that are not vulnerable to critiques around migration, diversity and the collapse of common values".
And while the exact outcome remains uncertain, he said the one certainty was Sweden's migration policies will now be reassessed.
A hung parliament?
With almost all districts having reported, the ruling center-left Social Democrats and Greens and their Left Party parliamentary allies had 40.6 per cent of the vote, while the opposition centre-right Alliance was at 40.3 per cent.
That gave the center-left 144 seats in the 349-seat parliament against 142 for the Alliance, suggesting weeks of uncertainty before a workable government can be formed.
The Sweden Democrats, a party with roots in the white supremacist fringe, won 17.6 percent and 63 seats, up from 12.9 per cent and 49 seats in the last election four years ago, the biggest gain by any party in Sweden’s parliament, the Riksdag.
The results were largely in line with the conventional opinion polls tracked by Reuters in the run-up to the election but well below some online surveys that had predicted the Sweden Democrats could become the largest party.
While the results also fell short of leader Jimmie Akesson’s predictions of 20 per cent of the vote or more, he told a party rally it was nevertheless the winner of the election.
“We will gain huge influence over what happens in Sweden during the coming weeks, months and years,” Akesson told party colleagues.
Akesson hopes his party, which wants Sweden to leave the European Union and freeze immigration, can play a decisive role in negotiations over forming a government.
He called on Ulf Kristersson, the center-right Alliance’s candidate for the premiership, to choose between seeking support from the Sweden Democrats for an Alliance government or to accept another four years of Social Democrat Prime Minister Stefan Lofven.
Kristersson called on Lofven to resign, but rebuffed Akesson.
“We have been completely clear during the whole election. The Alliance will not govern or discuss how to form a government with the Sweden Democrats,” he said.
The Sweden Democrats, shunned by all the other parties since entering parliament in 2010, have promised to sink any Cabinet that refuses to give them a say in policy, particularly on immigration.
Lofven said he would not resign and called for cross-party cooperation to resolve the political impasse.
“There is no side with a majority. Then it is only natural to work across the political divide to make it possible to govern Sweden,” Lofven said.
Le Pen hails Sweden Democrats' rise
Marine Le Pen of France's far-right Front National nonetheless hailed Sweden Democrats' projected rise, tweeting: "Yet another bad night ahead for the European Union. The democratic revolution in Europe is moving forward!"
The election will add to concerns in Brussels as the European Union enters campaign mode ahead of the European Parliament election in May, which could give more voice to eurosceptic groups and thwart efforts at closer EU integration.
The record levels of those seeking asylum in 2015 magnified worries about a welfare system in Sweden that many voters already believe is in crisis, even though refugee numbers have fallen sharply since then.
Lengthening queues for critical surgery, shortages of doctors and teachers and the failures of police to deal with inner-city gang violence have shaken faith in the “Swedish model”, built on a promise of comprehensive welfare and social inclusion.
Lofven had urged Swedes not to vote for what he called a "racist party" as he cast his ballot Sunday.
"It's... about decency, about a decent democracy. And the Social Democrats and a Social Democratic-led government is a guarantee for not letting the Sweden Democrats extremist party, racist party, get any influence in the government."
Akesson had labeled the vote a choice between immigration and welfare in a campaign that was unusually antagonistic.
Voting in central Stockholm, student Katze Collmar, 32, said the campaign had been “really unpleasant”.
“It feels like Sweden could take a step in this election that we won’t be able to recover from very easily.”
Lofven could hold on to power unless the Alliance accepts some kind of support from the Sweden Democrats, at a likely cost of the populists having more say over immigration policy.
But he would need to overcome decades of acrimony between the two blocs and get support from parties on the center-right, something they have ruled out.
Mattias, a Stockholm resident at an election night party in the city, said he was "extremely concerned" about the far right's steady climb.
"The election is between potential democracy and potential facism," he told AFP.
'Hostile to foreigners'
Anna Berglund, a 28-year-old lawyer who voted for the small Centre Party at a polling station in Stockholm's upmarket Ostermalm neighbourhood, agreed.
"I'm afraid we're becoming a society that is more hostile to foreigners."
The head of the four-party Alliance (the conservative Moderates, Centre, Liberals and Christian Democrats), Ulf Kristersson, told AFP on voting day his Alliance was the only option for change.
"We are the guarantee to oust the current government from power," he said.
With neither Lofven's "red-green" bloc nor his own Alliance standing a chance of winning a majority, Kristersson has said Sweden needs "a strong cross-bloc cooperation to isolate the forces... pushing for Sweden to withdraw from international cooperation".
'Time to talk to Sweden Democrats'
The final election results were due late Sunday, but the composition of the next government may not be known for weeks.
Lengthy negotiations will be needed to build a majority, or at least a minority that won't be toppled by the opposite side.
The opposition is intent on ousting Lofven, with some Moderates willing to go so far as to put an end to Sweden Demorats' pariah status and open negotiations with them.
That could prove fatal for the Alliance, with the Liberal and Centre parties repeatedly ruling out a deal with the far right.
None of the seven parties has been willing to negotiate with the Sweden Democrats, which first entered parliament in 2006 with 5.7 percent of votes.
"The problems in society that we warned of have grown bigger and worse and people agree with our view of reality," Sweden Democrats parliamentary group leader Mattias Karlsson told SVT.
"When the same party time and again increases, and the other parties stand still, then you have to listen to that part of the population that is voting for this party. It's time to take responsibility and talk to the Sweden Democrats," he said.
In an interview with AFP during the campaign, Akesson stressed he would "lay down his terms" after the election, citing immigration policy, crime-fighting and health care as priorities.