George W. Bush sought to boost the Republican presidential campaign of his brother Jeb Bush Monday in South Carolina ahead of this week's primary in the early voting state, endorsing his character while also striking against Donald Trump. The former president told an audience at a campaign rally in Charleston that Americans don't need a president who "inflames our frustration," which was taken as a clear swipe at Trump, according to the Associated Press.

"The presidency is a serious job that requires sound judgment and good ideas, and there's no doubt in my mind that Jeb Bush has the experience and the character to be a great president," Bush told the crowd while surrounded by his wife Laura, Jeb and South Carolina Sen. Lindsey Graham, The Los Angeles Times reported.

Bush, who has always been highly popular in South Carolina, encouraged voters to resist Trump's bombastic rhetoric without ever mentioning his name. "Strength is not empty rhetoric, it is not bluster," Bush said, adding that it is rather based on "integrity and character."

"In my experience, the strongest person usually isn't the loudest in the room," he added, according to Reuters.

The former president acknowledged the feelings among voters that Trump is tapping into, saying, "I understand that Americans are angry and frustrated but we do not need someone in the Oval Office who mirrors and inflames our anger and frustration," according to NJ.com. "We need someone that can fix the problems that cause our anger and frustration, and that's Jeb Bush."

However, Bush also called on voters to choose his brother not just because he is the opposite of Trump in terms of temperament, but also because of his "core conscience."

"I've seen in my brother a quiet conviction and a core conscience that cannot be shaken, and my hope is that the people of South Carolina will see this as well," Bush said. "This is a serious election for a serious job, so please welcome a serious and thoughtful candidate, a good man, a man I am proud to call my big little brother Jeb Bush."

In recent polling, Trump leads in South Carolina and Bush is in fifth place in the six-person Republican field. In an average of recent polls compiled by RealClear Politics, Trump has 36.3 percent support, with Ted Cruz in second with 17.8 percent support. Bush, who is only ahead of Ben Carson, has 9.3 percent support.