Donald Trump says a pact formed by his two rivals for the Republican presidential crown is a desperate act.
He lambasted Texas Senator Ted Cruz and Ohio Governor John Kasich over the alliance they announced late on Sunday.
Under their plan, Mr Cruz and Mr Kasich will give each other a free run in state primary votes next month.
Before then, five US states go to the polls on Tuesday, when Mr Trump is expected to tighten his grip on the nomination.
He has a clear lead in party delegates but may still fall short of the 1,237 needed to win outright.
If he does not reach that figure, the vote will go to a contested convention – where delegates are free to back another candidate. A different nominee like Mr Cruz or Mr Kasich may emerge.
On Monday, Mr Cruz defended the deal, saying it was “great for Indiana and great for the country”.