(AP) -- In an extraordinary display of Republican division, the Republican Party's 2012 presidential ticket took on its 2016 front-runner on Thursday as Mitt Romney joined the escalating charge to stop Donald Trump.

Romney, the GOP's presidential nominee four years ago, unleashed a public plea in the strongest terms for Republican voters to shun the former reality television star for the good of country and party. And as the former GOP nominee spoke in Utah, his 2012 running mate, House Speaker Paul Ryan, charged during a Capitol Hill press conference that "conservatism is being disfigured" by some of Trump's ideas and statements.

"His is not the temperament of a stable, thoughtful leader. His imagination must not be married to real power," Romney declared during a rare public appearance, calling Trump "a phony" who is "playing the American public for suckers."

Underlying the remarkable criticism was a bleak reality for panicking Republican officials: beyond harsh words, there is little they can do to stop Trump's march toward the Republican presidential nomination. Party elites were pouring over complicated delegate math, outlining hazy scenarios for a contested convention and even flirting with the long-shot prospect of a third party option.

Romney, too, seemed to embrace a long-shot approach to deny Trump the delegates necessary to secure the nomination. He did not call on Republicans to unify behind a single Trump alternative, but outlined a plan to divide the electorate and force a contested national convention in July.

"Given the current delegate selection process, this means that I would vote for Marco Rubio in Florida, for John Kasich in Ohio, and for Ted Cruz or whichever one of the other two contenders has the best chance of beating Mr. Trump in a given state," Romney said.

Before the speech, Trump dismissed Romney as "a stiff" who "didn't know what he was doing" as the party's candidate in 2012 and blew a chance to beat President Barack Obama. "People are energized by what I'm saying" in the campaign and turning out in remarkable numbers to vote, Trump told NBC's "Today."

The back-and-forth comes as Republican candidates prepared for the first post-Super Tuesday debate, scheduled for Thursday in Detroit, with Trump coming under increasing pressure from his party as he fights for the majority of delegates needed to win the nomination.

Thursday's clash came four years after the two men stood side by side in Las Vegas, with Trump saying it was a "real honor and privilege" to endorse Romney's White House bid. Accepting, Romney said it was a "delight" to have Trump on his side and praised him for ability to "understand how our economy works and to create jobs for the American people."

Romney on Thursday attacked Trump's temperament, his business acumen, and his ability to keep America safe.

"If we Republicans choose Donald Trump as our nominee, the prospects for a safe and prosperous future are greatly diminished," he said.

"Trump is directing our anger for less than noble purposes," Romney added. "This is the very brand of anger that has led other nations into the abyss."

Giving his party's 2012 presidential nominee the back of his hand, Trump turned his sights on the general election. His campaign reached out to House Speaker Paul Ryan's office to arrange a conversation between the two men, and urged Republican leaders to view his candidacy as a chance to expand the party.

During his Capitol Hill press conference, Ryan dismissed comments Trump told the world earlier in the week that if the Wisconsin Republican doesn't get along with him, Ryan would "pay a big price."

"I just laughed out loud," Ryan told reporters. "Sometimes, reality is stranger than fiction around here these days. I don't really think anything of it."

The two are among a group of party leaders growing increasingly critical of Trump. The GOP's 2008 nominee, Arizona Sen. John McCain, released a statement following Romney's speech, saying he echoes the "many concerns about Mr. Trump's uninformed and indeed dangerous statements on national security issues that have been raised by 65 Republican defense and foreign policy leaders."

Trump padded his lead with victories in seven Super Tuesday contests, with Cruz claiming three states and Florida Rubio picking up his first victory of the 2016 race.

Despite Trump's strong night, he was not yet on track to claim the nomination before the party's national gathering in July, according to an Associated Press delegate count. He has won 46 percent of the delegates awarded so far, and he would have to increase that to 51 percent in the remaining primaries.

GOP strategists cast March 15 as the last opportunity to stop Trump through the normal path of winning states and collecting delegates. A win for Rubio in his home state of Florida would raise questions about Trump's strength, as could a win for Kasich, Ohio's governor, on his home turf.

The GOP mayhem contrasted sharply with a clearer picture on the Democratic side, where Hillary Clinton was drawing broad support from voters and her party's leaders. Rival Sen. Bernie Sanders vowed to keep up the fight, though his path to the nomination has become exceedingly narrow.

The Associated Press has asked Republican governors and senators if they would support Trump if he becomes the party's nominee. Of the 59 respondents, slightly fewer than half could not commit to backing him in November.

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Associated Press writers Julie Pace, Kathleen Hennessey, Andrew Taylor, Julie Bykowicz, Stephen Ohlemacher, Alan Fram, and Donna Cassata contributed to this report from Washington.