Donald Trump could face a stiffer challenge securing 1,237 delegates to win the GOP presidential nomination and avoid a brokered convention because forces close to Marco Rubio are involved in the Ted Cruz–John Kasich alliance to block his path, sources said Monday.

“Little Marco ain’t so little anymore,” one Rubio insider told The Post, referring to Trump’s derisive nickname for the Florida senator.

When he was still in the race, Rubio collected 171 delegates — whom he can now withhold from Trump at the GOP convention.

A GOP strategist who backed Rubio was involved in talks that led to the alliance between Cruz and Kasich to derail Trump, the source said.

Kasich has agreed not to campaign in the May 3 Indiana primary to give Cruz a better chance. In return, Cruz is sitting out primaries in New Mexico and Oregon.

“It’s now going to be a more interesting fight. Cruz still has a shot to win if he can deny Trump triumphs in Indiana, Nebraska, Oregon and New Mexico,” said political analyst Dick Morris.

Pro-Cruz and anti-Trump super PACs, including the Club for Growth, are spending at least $2 million combined in ads in Indiana, which has 57 delegates. The winner of the primary gets 30 delegates, with the rest allocated by congressional district.

“It is very significant Kasich is pulling out of Indiana,” Cruz said on “The Laura Ingraham Show” Monday.

But others, including Trump, said the gambit is politics at its worst.

“How do you nominate a guy who lost [to me] by 4 to 5 million votes? We’re going to pick a guy who got creamed? . . . You would have a revolt,” Trump said while stumping at West Chester University in Pennsylvania.

“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 added in a statement.

Pennsylvania GOP chairman Rob Gleason called the Cruz-Kasich alliance a “weak move” that was “too little, too late.”

“It’s going to be Hillary vs. Trump in the general election. That’s what the people want,” Gleason said.