He referred to "an insurgent group" trying to deny him delegates at the July convention.
Republican hopes are fading for a new, "more presidential" Trump.
Republicans are looking for assurances that Donald Trump can maintain the discipline needed to stay on message as he prepares for a bruising general election run-up against Hillary Clinton.
Trump appeared on ABC's "This Week" and NBC's "Meet the Press."
This is the last chance for Cruz.
Republicans share a palpable fear that Trump would alienate Hispanics, minorities, independents and women, driving them to vote Democratic in November and costing the GOP the presidency.
Rubio racked up endorsements from prominent Republicans.
Despite Obama's reassurances, Republicans say Obama has failed to grasp the severity of the risk.
Trump cheered this week's ruling that said President Obama could not continue with his plan to overhaul the country's immigration system.
It's an uncomfortable moment for Republicans, who want more votes from the surging Latino population.