Donald Trump's Republican rivals briefly join forces to stop him

In an extraordinary move, Donald Trump's Republican rivals late Sunday announced plans to co-ordinate primary strategies in upcoming states to deprive the Republican front-runner of the delegates needed to win the Republican nomination.

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The arrangement does not address the five Northeastern state set to vote on Tuesday, where Trump is expected to add to his already overwhelming delegate lead. Kasich and Cruz had already retreated to Indiana, which holds its primary on May 3. Yet the shift offers increasingly desperate Trump foes a glimmer of hope in their long and frustrating fight to halt the former reality television star's unlikely rise.

Trump win 'a sure disaster' 

"Having Donald Trump at the top of the ticket in November would be a sure disaster for Republicans," Cruz's campaign manager, Jeff Roe, said in a statement explaining the new plans. "Not only would Trump get blown out by Clinton or Sanders, but having him as our nominee would set the party back a generation."

Added Kasich's chief strategist, John Weaver, "Our goal is to have an open convention in Cleveland, where we are confident a candidate capable of uniting the party and winning in November will emerge as the nominee."

The announcement marks a sharp reversal for Cruz's team, which aggressively opposed co-ordinating anti-Trump efforts with Kasich as recently as late last week. And it only applies to Indiana, Oregon and New Mexico — three of the 15 states remaining on the Republican primary calendar.

Yet the development underscores a bleak reality for the billionaire businessman's Republican foes: Time is running out to stop him.

Trump responded on Twitter shortly before midnight, accusing the pair of being desperate. 

The announcement came less than 48 hours before voting begins across five Northeastern states where the New York billionaire leads in many polls. Trump campaigned Sunday in Maryland, which will vote on Tuesday along with Rhode Island, Connecticut, Pennsylvania and Delaware.

Speaking to several thousand people in an airplane hangar in Hagerstown, Md., Sunday evening, Trump stressed repeatedly that he expects to win the 1,237 delegates needed in the first round of voting to stave off a contested convention.

"I only care about the first. We're not going for the second and third and fourth and fifth," said Trump.

System is 'rigged,' says Trump

As Kasich backs out of Indiana, Cruz promised to not compete in primary contests in Oregon on May 17 and New Mexico on June 7.

"We will focus our time and resources in New Mexico and Oregon, both areas that are structurally similar to the Northeast politically, where Gov. Kasich is performing well," Weaver said.

As recently as three days ago Kasich's campaign announced investments in Indiana, including the opening of two offices and the creation of a campaign leadership team. His campaign on Sunday night cancelled a town hall and gathering in Indianapolis scheduled to watch the results of Tuesday's primaries.

Like Cruz's campaign, Kasich's campaign encouraged allied super PACs and other outside groups to "honor the commitments."

In recent weeks, Trump has repeatedly denounced the Republican's presidential nominating system as "rigged." That criticism is likely to intensify in the coming days.

Democrats 'are going to be unified' 

There was far less drama on the Democratic side Sunday.

Underdog Bernie Sanders rallied thousands of voters in two New England states, seeking momentum even as he offered mixed signals on how hard he would push his differences with front-runner Hillary Clinton.

The Vermont senator largely steered clear of Clinton at a Rhode Island park, but hours later ramped up his critique before more than 14,000 supporters in New Haven, Connecticut. Sanders reiterated his call for Clinton to release transcripts of lucrative Wall Street speeches she delivered after leaving the State Department in early 2013.

"This campaign, unlike Secretary Clinton's, has not raised $15 million from Wall Street and millions more from other special interests," he said as the crowd booed at the mention of Clinton's name.

Clinton eyed victories in four or five of Tuesday's contests, which would all but cripple Sanders' White House bid.

The former secretary of state went to two Philadelphia church services attended largely by African-Americans ahead of the primary in Pennsylvania, Tuesday's top delegate prize. She declined to attack her Democratic rival by name in the morning appearance and a subsequent stop in Bridgeport, Connecticut, focusing on the GOP candidates.

Democratic National Committee Chair Debbie Wasserman Schultz downplayed tensions between Sanders and Clinton, whose rivalry has become increasingly nasty in recent weeks.

"Regardless of the intensity of what's played out here ... we are going to be unified," she declared.