Recent polls suggest they could win the election, or come second to the centre-right Liberals (VVD).
The polls point to a neck-and-neck race between the SP and the liberal VVD for top spot.
The VVD and CDA are obvious bedfellows, but they will need further support if they are to govern.
The liberal VVD party has gone furthest, pledging cuts that should bring the budget into surplus within two terms.
The VVD, which has never before led a Dutch government, finds itself top of the polls, with 25% support.
Support for the VVD, which is toying with euroscepticism, is broadly holding up, but the CDA is losing ground.
Under the deal, VVD leader Mr Rutte would become prime minister, forming a cabinet with the CDA, led by Maxime Verhagen.
Mr Roemer and the VVD leader, Mark Rutte, who is also caretaker prime minister, have yet to take part in a debate.
The Socialists may get 31 seats, just ahead of Mr Rutte's VVD.
Mr Rutte's liberal VVD party and the centre-left Labour party secured the coalition deal after coming out on top in 12 September elections.
Mr Wilders' party took heavy losses in the election, while VVD claimed overall victory with Diederik Samsom's Labour Party coming a close second.
Correspondents say Mr Rutte's VVD has stepped up its Eurosceptic rhetoric in a bid to woo right-wing voters, emphasising its pragmatic approach to Brussels.
The Liberal party (VVD) and the Christian Democrats (CDA) have 52 seats between them in the 150-seat parliament and propose to form a minority government.
Mark Rutte, leader of the VVD, has indicated that the differences between his party and Labour would make it difficult for the two to govern together.
The VVD, for example, is campaigning on a promise to limit the flow of unskilled immigrants into the country and to restrict newcomers' access to social services.
Both the centre-left Labour party and the centre-right VVD party won a sufficient number of votes between themselves to rule with a majority in the lower houses of parliament.
BBC: Netherlands election analysis: Dutch show faith in Europe
His party came third in June's general election, winning 15% of the vote, and will now prop up a minority government of the liberal VVD with the centre-right Christian Democrats.
An ardent campaigner for the rights of Muslim women, Ms Hirsi Ali started her political career as a Dutch social-democrat, and then moved to the free-market and right-wing liberal party, the VVD.
Speaking after a cabinet meeting on Monday morning involving Mr Rutte's liberal VVD and the Christian Democrats (CDA), Finance Minister Jan Kees de Jager said they would "show the financial market and Dutch people that we can also put forward a solid financial policy".
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