US Senator Ted Cruz refused to endorse Republican presidential nominee Donald Trump at the party's convention, drawing angry jeers from Trump supporters.
It also shattered the facade of party unity that has been carefully built up in Cleveland this week.
Anti-Trump Republican delegate Ken Cuccinelli told Reuters he escorted Mr Cruz's wife, Heidi, off the floor of the Republican National Convention out of concern for her safety following her husband's speech.
For the third day in a row, what was supposed to be a carefully scripted show of unity was thrown into turmoil by unexpected events that have raised questions about whether the party can unite around Mr Trump to defeat the presumptive Democratic nominee, Hillary Clinton, in the 8 November election.
Mr Cruz, who came in a distant second to Trump in the race for the Republican nomination, stopped short of endorsing Mr Trump after a bitter and personal campaign and mentioned him only once, drawing boos and repeated chants of "We want Trump."
Mr Cruz began his speech saying: "I want to congratulate Donald Trump on winning the nomination last night."
Later in the speech, he urged: "Please, don't stay home in November. Stand, and speak, and vote your conscience, vote for candidates up and down the ticket who you trust to defend our freedom and to be faithful to the constitution."
Some critics saw the appeal for people to vote with their conscience as a vote of no-confidence in Mr Trump.
US Senator Ted Cruz refused to endorse Donald Trump at the GOP, drawing angry jeers from Trump supporters.https://t.co/c8ISPTIgTm
— RTÉ News (@rtenews) July 21, 2016
Republican strategist Eric Fehrnstrom, who is not affiliated with any campaign, tweeted: "'Vote your conscience' was the rallying cry of the Never Trump movement. For Cruz to bring that message into #RNC hall was a colossal error."
"It's taken me about 30 minutes to calm down and stop shaking with anger," said Erik Layton, an alternate delegate from California who had shouted: "Go home" at Mr Cruz after his speech.
"I just don't know why Cruz did this. It baffles my mind."
Mr Trump made his entrance to the convention hall near the end of Mr Cruz's speech, applauding Mr Cruz's remarks but, by his appearance, drawing attention away from his former rival.
In a tweet after the convention adjourned for the night, Mr Trump wrote that Mr Cruz broke a promise they both had made to endorse the party's White House choice.
Wow, Ted Cruz got booed off the stage, didn't honor the pledge! I saw his speech two hours early but let him speak anyway. No big deal!
— Donald J. Trump (@realDonaldTrump) July 21, 2016
During the campaign for the party's nomination, Mr Trump insulted Mr Cruz's wife's looks and suggested the Texan's father was with John F Kennedy's assassin just before the president was shot in Dallas in 1963.
Mr Cruz, who as a Tea Party conservative in the Senate angered many mainstream Republicans in Congress and spearheaded tactics that led to a government shutdown over the federal budget, called Mr Trump a "serial philanderer" and a "narcissist" during the campaign.
A Cruz adviser who asked to remain anonymous said Mr Cruz anticipated a backlash from the crowd if he did not endorse Mr Trump.
"We knew people were going to be mad if he didn't say the words, but he congratulated him and called for unity behind common values. He expected people to not be thrilled about this," the adviser said.
Today Mr Cruz said he is "not in the habit of supporting people who attack my wife and attack my father".
He would not say if he will vote for Mr Trump in the presidential election, but he did say "I'm not voting for Hillary".
Mr Trump won the party's nomination on Tuesday with 1,725 delegates, followed by Mr Cruz with 475 delegates.
.@tedcruz is booed off stage after failing to endorse @realDonaldTrump. Hear more @rtenews @morningireland #RNCinCLE pic.twitter.com/XYwfulduog
— Caitriona Perry (@CaitrionaPerry) July 21, 2016
