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”.
Can the pact work? – Anthony Zurcher, BBC News, Washington
This announcement comes just days before Mr Cruz and Mr Kasich are likely to receive a thorough drubbing in a handful of states in the mid-Atlantic, including delegate-rich Pennsylvania.
By Wednesday morning Mr Trump could have put considerably more distance between himself and his two opponents.
So this accord may be an early effort to push the focus past the next round of voting and on to more friendly contests.
Indiana is shaping up to be a pivotal battleground. A recent poll shows Mr Trump with a comfortable lead in a three-way race that narrows considerably if Mr Kasich is taken out of the equation.
Given the rules in Indiana – 30 delegates to the candidate who wins a statewide plurality and three delegates to the top finisher in each of the state’s nine congressional districts – every bit of help Mr Cruz can get to edge past Mr Trump will be invaluable.
Read Anthony’s piece in full
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.
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.
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Five states go to the polls on Tuesday – 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.