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 which they 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 1237 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".
Speaking in Indiana, the Texas senator said Mr Trump winning the nomination would hand the White House to Democratic front-runner Hillary Clinton in November's election.
But in a series of tweets on Monday, Mr Trump, a New York businessman with no experience of elected office, said this was collusion from two weak candidates.
Wow, just announced that Lyin' Ted and Kasich are going to collude in order to keep me from getting the Republican nomination. DESPERATION!— Donald J. Trump (@realDonaldTrump) April 25, 2016
Lyin' Ted and Kasich are mathematically dead and totally desperate. Their donors & special interest groups are not happy with them. Sad!— Donald J. Trump (@realDonaldTrump) April 25, 2016
And in a statement, he said: "It is sad that two grown politicians have to collude against one person who has only been a politician for ten months in order to try and stop that person from getting the Republican nomination."
Mr Trump has waged war on the Republican National Committee over the process by which delegates are allocated, saying the system is "rigged" against him.
The five states that go to the polls on Tuesday are Connecticut, Delaware, Maryland, Pennsylvania and Rhode Island.
Under the Cruz-Kasich plan, Mr Cruz will cut campaigning in the Oregon and New Mexico primaries and Mr Kasich will give Mr Cruz a "clear path" in Indiana.
Indiana and Oregon vote next month, with New Mexico to follow in June.
In the Democratic race, Mrs Clinton will be looking to tighten her grip on the nomination after her big New York win, but Vermont Senator Bernie Sanders says he still has a path to victory.