STOCKHOLM (Reuters) - Sweden’s center-left bloc holds a small but stable lead over the center-right Alliance ahead of Sunday’s election, two opinion polls showed on Tuesday, but forming a government after the vote may be extremely difficult.

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The Social Democrats with junior coalition partner the Greens - together with the Left Party - hold a lead of around 4 percent over the center-right opposition bloc, opinion polls by Kantar Sifo and Ipsos published by newspapers Svenska Dagbladet and Dagens Nyheter showed.

All the mainstream parties have promised to exclude the Sweden Democrats, a party with roots in the far-right fringe, polling 16-18 percent.

In turn, the Sweden Democrats have vowed to bring down any government that does not give them a say over immigration policy, leaving no clear successor to the current coalition.

“It is likeliest we will see a single-party, Social Democrat government,” said Nick Aylott, a political scientist at Sodertorn University.

Prime Minister Stefan Lofven hopes to hold on to power by persuading center-right Alliance members, the Centre and Liberal parties, to support his center-left government rather than make a deal with the Sweden Democrats.

“Even if we are bigger (than the Alliance) I will seek some form of cooperation across the political divide,” he told Reuters on Monday.

He said that could mean organized cooperation in parliament or other forms of collaboration.

For the Centre and Liberal parties, the option of a Social Democrat government may be more palatable than an Alliance government that is smaller than the center-left and is supported by the Sweden Democrats.

The Moderates, the biggest party in the Alliance, have said they will try to oust Lofven in a mandatory vote on the prime minister’s post after the election.

But the party is facing its worst result since the turn of the century. If their recent downward trend continues, it could be the worst since the early 1970s. That would undermine leader Ulf Kristersson’s claim on the premiership, which would also rely on support from the Sweden Democrats.

After a turbulent period in government, the Green party may opt to step out of the limelight. The party nearly left the coalition in 2015 over the Social Democrats’ plan to tighten immigration policy. The Social Democrats have said stricter asylum rules are here to stay.

The Social Democrats got around 26 percent in the two polls. The Moderates around 17 percent, roughly the same as the Sweden Democrats.

Graphic: tmsnrt.rs/2LmSZFD