(Replaces exit poll with preliminary results)
By Toby Sterling and Bart H. Meijer
AMSTERDAM, March 21 (Reuters) - The Dutch government will lose its majority in the Senate after provincial elections on Wednesday, preliminary results showed, as voters flocked to a new populist party two days after a possible terrorist attack in the city of Utrecht.
If confirmed, the result would mean that Prime Minister Mark Rutte's centre-right coalition will need to seek outside support to pass legislation.
First estimates based on 60 percent of votes cast showed the new Forum for Democracy will become the largest party in the Senate, in a tie with Rutte's conservative VVD Party, on its first try.
Following the lead of U.S. President Donald Trump, Forum for Democracy leader Thierry Baudet opposes immigration and emphasises "Dutch first" cultural and economic themes.
Pollsters had for weeks predicted Rutte's centre-right coalition would lose its Senate majority. But experts, including pollster Maurice de Hond, said the Utrecht attack, which killed three people, appeared to boost turnout most among opponents of immigration.
Rutte's VVD is forecast to fall to 12 seats, from 13 in the 75-member Senate, and his coalition as a whole will fall from 38 seats to 31. The Forum for Democracy is also estimated to take 12 seats. The country's Electoral Council will publish final results on March 25.
The Senate, the upper house of Dutch parliament, is responsible for reviewing measures passed by the more powerful lower house.
Forum for Democracy leader Baudet shocked establishment parties by saying the Utrecht attack was the result of lax government immigration policies. Others suspended campaigning.
The motive of the 37-year-old Turkish-born man arrested on suspicion of carrying out the attack on a tram is not yet known. .
"You can tell what's going on anyway," Baudet told a rally. "This is a combination of an honour killing and a half-terrorist motive."
The Dutch economy has been one of Europe's best performers under successive Rutte-led governments, but resentment over early 2010s austerity programs lingers. Recent debate has focused on funding the government's plans to meet international goals on climate change.
Many supporters of the conservative parties in Rutte's coalition are sceptical of spending on climate change. Left-leaning voters feel not enough is being done and defected to the pro-environment Green Left party, which also booked big gains on Wednesday.
Rutte is expected to look to the Green Left and Labour parties for outside support once the new Senate is seated in May. (Reporting by Toby Sterling; Editing by Cynthia Osterman and Clarence Fernandez)
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