Texas Senator Ted Cruz was booed as he failed to endorse Donald Trump as the Republican nominee, during a speech at the party convention in Cleveland.
Mr Cruz only went as far as congratulating the man who was his bitter rival in the primary contests.
Angry chants of "We want Trump!" and "Endorse Trump!" grew louder as the senator came to the end of his speech.
Indiana Governor Mike Pence then took centre stage to give his vice-presidential acceptance speech.
Mr Pence, 57, praised Mr Trump's record as a businessman and said the the country only had "but one choice" in November.
"It's change versus the status quo, and when Donald Trump is president, the change will be huge," he said.
But Mr Cruz is likely to steal the headlines after stoking the rage of the convention, to such an extent that his wife Heidi had to be escorted from the floor.
At the scene - Anthony Zurcher, BBC News, Cleveland
You could tell the exact moment the crowd at the Republican convention turned against Ted Cruz. When the Texas senator told the audience not to stay home in November, everyone expected an endorsement of the new presidential nominee to follow.
"Do it, Ted!" shouted one man near the front of the stage in the California delegation. Instead, Mr Cruz implored those listening to "voter your conscience". Then things got ugly.
Much of the crowd started booing. Some chanted "Trump! Trump! Trump!"
By the time the senator finished his speech, the damage - to the efforts to display party unity and possibly to Mr Cruz's political future - was done.
Mr Cruz has a Senate election in two years. Two years later, in 2020, he could make another try for the presidency.
If Mr Trump goes down in flames, he could be the principled candidate who avoided getting burned. Or it could be his political bridges to the movement that Mr Trump represents that just went up in flames.
In his speech, he told supporters to "vote [with] their conscience" and not to stay at home for the general election.
Mr Cruz finished second to Mr Trump in the delegate count during a campaign that featured personal attacks by the New York businessman on Mr Cruz's wife and father.
The boos that drowned him out stopped when Mr Trump entered the convention hall before Mr Cruz had finished speaking.
Other highlights from Wednesday evening:
shouts of "Lock her up!" directed at Democrat Hillary Clinton rang out from the floor for the second night running
Florida Senator Marco Rubio gave a full-throated support of Mr Trump via video
there were 17 arrests outside the arena after a scuffle broke out from a flag-burning in the streets
America deserved better than Mrs Clinton, said Wisconsin Governor Scott Walker
Eric Trump talked effusively about his father
Mr Trump later said he knew Mr Cruz would not endorse him but let him speak anyway.
New Jersey Governor Chris Christie said his actions were "awful" and New York Congressman Peter King called the speech "disgraceful".
The drama came at the end of the day when the Republicans tried to finally put the plagiarism scandal behind them.
Melania Trump faced accusations of copying after it was discovered that lines in her remarks matched almost word for word a convention speech delivered by Michelle Obama in 2008.
After initial denials from the campaign, Meredith McIver, a Trump Organisation employee, admitted her role in writing the speech and apologised for the "chaos I have caused".
A convention - all you need to know
1. What's the point? Each party formally nominates its candidates for president and vice president, and the party unveils its party platform, or manifesto.
2. Who is going? There are 2,472 delegates attending, selected at state and congressional district conventions, and representing each US state and territory. Plus 15,000 journalists and thousands of other party grandees, lawmakers and guests.
3. Who isn't going? Some senior figures who don't like Donald Trump have stayed away, including two ex-presidents named Bush, former nominee Mitt Romney and Ohio Governor John Kasich.
4. What's the schedule?
Wednesday - VP nominee Mike Pence
Thursday - Donald Trump, introduced by daughter Ivanka
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