But the two parties would have to rely on Mr. Wilders to get important legislation through, including austerity cuts expected to total about $24 billion. If the deal comes to fruition, Mr. Wilders will follow in the footsteps of Denmark’s populist, anti-immigration Danish People’s Party, which has stayed out of government but supported a minority center-right administration.

On Wednesday, Dutch center-left parties criticized the proposed new alliance during a special hearing in Parliament for which deputies were called back from summer vacation. During the debate, the Labor Party leader, Job Cohen, described Mr. Wilders as “the puppet master pulling the strings behind the scenes, with all the advantages and none of the responsibility,” Radio Netherlands reported.

Some analysts also contended that such an arrangement would be ideal for Mr. Wilders, a clearly recognizable figure with bleached-blond hair who lives under 24-hour protection after a succession of death threats.

“The risky scenario for Wilders is becoming part of the government and being responsible for policies,” said Dick Houtman, a political sociology professor at Erasmus University in Rotterdam. “On the other hand, if it becomes clear that he doesn’t want to be part of government, then he loses credibility and people will stop supporting his party.

“The fear is that you will give Wilders lots of influence over the government, but on the other hand, he will still be able to say what he wants,” Professor Houtman said. “It’s a dream outcome for Wilders.” Mr. Wilders’s more far-reaching anti-Islamic ideas are unlikely to be included in any agreement, the professor said, though he added that the last government’s immigration policy was already tough.