Donald Trump will take the stage Thursday to accept the Republican Party's nomination to run for president on the final day of a national convention that many hoped would unify the party, temper the candidate's image and give him a boost in the polls.

But the first two days of the convention, that Trump once promised would be a showbiz extravaganza like nothing before it, have been a series of stumbles that have raised the stakes for what already was the most important speech he's ever likely to make.

Republican strategist Ed Rollins, who is advising a “super PAC” supporting Trump, said he can't afford to any mistakes at this point, the New York Times reported.

“His speech is the whole game,” Rollins told the Times. “Viewers have to watch it and say, ‘There is the next president of the United States.’”

Other political experts said the four-day convention in Cleveland hasn't accomplished much in positioning Trump to take on his Democratic foe, former Secretary of State Hillary Clinton.

“The first two days have been sort of a number of incidents that I think have stepped on what I’m sure Trump would have wanted to be a much cleaner and stronger message about his candidacy,” said Patrick Egan, a political science professor at NYU. 

The missteps at the convention have overshadowed the message, beginning with the last-ditch effort by the Never Trump delegates to try to deny him the nomination in a rules fight on the first day. That display of party dischord was followed by the embarrassment of Melania Trump, the candidate's wife, being accused of plagiarism for lifting part of her speech from one given by Michelle Obama in 2008.

The lack of A-list speakers, and up and coming Republicans has also been a drawback, especially after Trump promised a lineup of all-stars and winners, including Patriots quarterback Tom Brady, who decline the invitation.

Heath Brown, assistant professor at CUNY City University of New York Grad Center and John Jay College said the lack of A-listers isn’t unusual for a GOP event, but the lack of rising and established Republican stars is a glaring omission.

“You don’t have anyone from the most significant Republican group over the last generation,” he said. “Nobody from the Bush family is there; in addition, you don’t have anyone from the Romney group as well, so I think that’s what’s really remarkable.”

“I do sense that there is a bit of breath-holding at the convention,” Brown added. 

Brown said in order for Trump to bring the RNC to a crescendo, he’ll have to rely on tips from Ronald Reagan’s 1980 acceptance of the nomination.

“The first thing [Reagan] did in 1980 was he spoke with a level of specificity about his agenda and Trump really hasn’t done that before,” Brown said. "I suspect he’s going to try to speak as Ronald Reagan did in 1980 beyond the diehard backers of his party and his candidacy to those who might not be informed about what Trump is all about or might not be paying as much attention and they need to be convinced in a slightly different way.”

-Joel Millman contributed to this report.