AMSTERDAM (AP) ― With less than a week to go before national elections, an abrupt rise in the popularity of the brainy new leader of the Netherlands’ Labor Party, Diederik Samsom, has conservative Prime Minister Mark Rutte suddenly fighting to hang on to his slim lead in the polls.

Neither candidate is seeking to radically alter the Netherlands’ relationship with Europe, but Samsom, a former Greenpeace activist with an engineering degree in nuclear physics, has said he views Rutte’s emphasis on austerity as misguided. If elected, he would de-prioritize spending cuts in the short term, investing more in education.

His slogan for Wednesday’s vote: make the Netherlands “stronger and more compassionate.”

The reason behind Samsom’s rise is no mystery. Labor had been lagging in the polls until the first televised debate by the top four candidates last month, in which Samsom was able to position himself as the most reasonable alternative to Rutte.

The other top candidates were far-left Socialist Leader Emile Roemer ―a party with communist roots that has never participated in government ― and populist Geert Wilders, known for his anti-Islam stances and his calls for the Netherlands to withdraw from the eurozone.

Samsom grasped the opportunity with both hands, displaying an understanding of policy nuances, acting humble, and criticizing his opponents for making unrealistic pledges. That style has continued to serve him well.

In an interview with De Telegraaf newspaper published Saturday, Rutte called Samsom “a danger for the Netherlands,’’ and criticized his economic policies.

Rutte may be right to worry about Samsom’s rise. According to pollster Maurice de Hond in a poll released Friday, a growing number of voters now view Samsom as a potential prime minister.

Socialist Party leader Roemer is widely liked but has not been seen as having the stature needed to rule.

One popular news website routinely refers to him as “Fozzie Bear.” Wilders’ voters are intensely devoted, but his anti-Europe, anti-Islam rhetoric is not getting wider traction. Both candidates have lost ground in recent polls.

De Hond said that when voters were asked “who do you want to be prime minister if you can choose between Mark Rutte and Diederik Samsom?’’ 42 percent of respondents chose Rutte, while 47 percent chose Samsom.

After a decade of right and center-right governments in the Netherlands, left-leaning parties command a majority in most opinion polls, though they encompass many small parties. With a record 40 percent of voters still undecided less than a week before the election, if progressives begin to see Samsom as having a chance to beat Rutte, they may throw their support behind him.

Although Samsom was only elected Labor leader in March, he is a reasonably well-known figure in the Netherlands.

He has served in parliament since 2003, and his ability to perform well on television should not have come as a surprise, as he has been a serial victor on celebrity television quiz programs. In 2005 and 2006 he won “The National News Quiz,’’ in 2008 he won “The National IQ Test’’ and the same year he won “The Big History Quiz.’’

His wife reportedly told him he had proved his point and should stop competing.

Samsom may also be benefitting from what voters don’t know about him. Before entering politics, he worked for Greenpeace, where he was arrested several times, though he does not have a criminal record.

In an uncomfortable interview with a women’s magazine in August he admitted being vain, unhappy about going bald, and having cheated on past girlfriends before meeting his wife. He is an atheist and a vegetarian.

“If you look at our platform, then the Socialist Party and Green-Left are our most obvious partners,’’ he said in the interview with RTL television Friday. But he left the door open to forming a government with any party, including Rutte’s, no matter how the vote turns out.

“The Socialists are saying they’ll never help the right reach a majority, and Mark Rutte is saying he’ll never help the left reach a majority,” he said.