Donald Trump called Gov. John Kasich and Sen. Ted Cruz’s plan to join forces against him an act of “desperation” in a series of tweets he fired off late Sunday. Though the #NeverTrump coalition is likely cheering the decision, the Republican candidate is not happy.

Trump continued bashing Cruz, Kasich on Twitter Monday.

On Sunday, both Republican campaigns released statements announced they would work strategically to win delegates in upcoming contests with their primary goal being to prevent Trump from winning the 1,237 delegates he needs to earn the party’s nomination.

 As TIME’s Zeke Miller reported Sunday, the plan has been in the making since late February:

The Kasich campaign first approached the Cruz campaign about coordinating their efforts after the Houston debate in late February, only to be rejected. Last week, at the spring meeting of the Republican National Committee in Hollywood, Fla., where both Cruz and Kasich and their top strategists made presentations, Cruz campaign manager Jeff Roe approached Kasich senior strategist John Weaver about cooperating in the coming races, according to a person familiar with the arrangement. The details were then sorted out in a series of telephone conversations over the weekend.

The Cruz campaign said in a statement it will focus on winning the Indiana primary, where the Kasich campaign has agreed to pull back. The Kasich campaign, in turn, will put time and resources into the contests in Oregon and New Mexico.

Trump’s campaign issued a statement on Sunday in which he suggested the activities of the campaign would be “illegal” in other industries.

“Collusion is often illegal in many other industries and yet these two Washington insiders have had to revert to collusion in order to stay alive,” Trump’s statement reads. “They are mathematically dead and this act only shows, as puppets of donors and special interests, how truly weak they and their campaigns are.”