Photo: The Canadian Press

Unmoved by harsh debate reviews, a defiant Donald Trump renewed his attacks against a former Miss Universe winner on Wednesday, showing no sign of making big changes to his message or debate preparation before his second face-off with Hillary Clinton.

The outspoken Republican nominee instead pressed ahead with an aggressive strategy focused on speaking directly to his white, working-class loyalists across the Midwest.

Democrat Clinton, meanwhile, pushed to improve her standing among younger voters with the help of the president, Sen. Bernie Sanders and other key allies, 48 hours after a debate performance that seemed to spark badly needed enthusiasm.

Those closest to Trump insisted the Republican presidential nominee was satisfied with Monday night's debate, even as prominent voices within his own party called for more serious preparation next time following an opening confrontation marked by missed opportunities and missteps.

"Why would we change if we won the debate?" former New York Mayor Rudy Giuliani, a key Trump ally and travelling partner this week, told The Associated Press.

The next debate is 11 days away.

While his plan forward is far from set, Trump is not planning to participate in any mock debates, although he is likely to incorporate what one person described as "tweaks" to his strategy.

Specifically, Trump is likely to spend more time working on specific answers and sharpening his attacks after spending much of the first meeting on defence, said that person, who spoke on condition of anonymity to discuss internal campaign strategy.

That may not be enough to satisfy concerned Republicans.

Former House Speaker Newt Gingrich said Trump should have been better prepared and he recommended that the candidate work harder with skilled coaches. He said, "What you need is people who are professional debaters."

South Carolina Sen. Lindsey Graham said simply, "The only advice I could give him, and take it for what it's worth: Prepare better."