Countries supporting Dutch Prime Minister Mark Rutte’s bid to lead NATO have been working behind the scenes to make sure the race is wrapped up by this summer, according to people familiar with the matter.

The decision needs to be unanimous among the 31 North Atlantic Treaty Organization allies and the Dutch leader is already the strong favorite to succeed current Secretary General Jens Stoltenberg when his term ends in October. Nevertheless, some countries in eastern Europe have yet to give Rutte their backing, the people said.

Rutte made an appearance at the World Economic Forum in Davos, Switzerland, last week when he urged European countries to spend more on defense, a common refrain from former U.S. President Donald Trump who is a strong contender to reclaim the White House in November. Rutte also backed U.S. and U.K. attacks against the Houthis earlier this month.