The motto is a not-so-thinly-veiled response to "I'm With Her," which supporters of likely Democratic nominee Hillary Clinton have trumpeted for months as she tries to become the country's first female president. Trump's word choice also underscores differences in the core messages at the heart of each campaign.
Republicans see Trump's new phrase as savvy branding that could help sharpen the contrast with Clinton by emphasizing his populist bent. But Democrats said the phrase flies in the face of Trump's well-known divisive rhetoric and predicted it will do little to repair his image among moderate voters.
The three words highlight one of the central questions surrounding Trump's candidacy as the general election takes shape: Can a candidate who has already polarized the electorate successfully broaden his appeal, or has a year's worth of incendiary remarks about minorities, women and leaders in both parties made that task impossible?
Vincent Harris, a veteran digital strategist who worked on Kentucky Sen. Rand Paul's presidential campaign, wrote in an email that Trump has a real opportunity to employ the phrase as part of a populist pitch on social media.
"Twitter is a wonderful tool to reach the political base and while Clinton is focused on using social media to boost herself, if Trump can make his digital operation about voters and the people, it will pay off come election time," Harris wrote.
Ida Woldemichael, a designer who came up with "I'm with Her" for the Clinton campaign, said it is “exciting” that others are drawing attention to the slogan and even co-opting it.
“The fact that the Trump team used it in their messages and flipped it, I don’t feel like it’s a slap in the face, because he already presents who he is by what he says," she said.
Denise Horn, a spokeswoman for the Clinton campaign was skeptical of the new Trump slogan. “They’re talking about 'I’m with you,' but you’re not with Muslims, or black people or LGBT people," Horn said. "Who are you with exactly?”
A Trump spokeswoman did not respond to a request for comment on the origin of "I'm with You" or elaborate on it.
In a Wednesday speech, Trump said Clinton "believes she is entitled to the office. Her campaign slogan is 'I’m with her.' You know what my response to that is? I’m with you -- the American people."
He followed up with a series of tweets featuring the hashtag "#Imwithyou." His campaign is also selling T-shirts online with Trump's picture above the words, "I'M WITH YOU."
In a regularly scheduled meeting on Capitol Hill Thursday, Trump campaign officials talked to supportive House members about trying to make the election a binary choice between Trump and Clinton, according to a person familiar with the talks.
During the primary, Trump railed against party elites and carried the mantle of aggrieved working class voters, running as an everyman despite his vast wealth. It worked.
Keith Appell, a Republican strategist who supports Trump, wrote in an email that the presumptive nominee should "absolutely stick with" his "I'm with You" line.
"It speaks to the populist nature of his campaign," he said. "One of the best attack lines against Hillary is that she’s not really one of us, she’s an elitist who literally acts as if she’s above the law; that she offers nothing tangible to improve people’s lives or make them feel more secure; that she’s really just running for herself without any meaningful rationale."
Woldemichael, 34, is a graphic designer who worked for the Clinton Foundation before joining the campaign about a year ago. She described "I'm With Her" as “a grassroots movement" that started with an email contest for Clinton supporters early in the campaign.
Woldemichael came up several slogans to choose from and "I'm with Her" was the winner. It topped other options, such as the more bland, “Team Hillary.” Now, "I'm With Her" is all over Clinton buttons and t-shirts, in addition to being used by supporters on social media.
She said the thinking behind the phrase was "what separates Clinton from everyone else? It was the simple fact that she was the only Democratic female running for the nomination. That was something that was purely unique that we could say about our candidate."
Trump is arguing that he is better for women than Clinton. He is also underscoring an argument that his hard-line posture on the Islamic State terrorist group makes him a better ally to the LGBT community, since the Islamic State treats gay people brutally.
It's all a part of Trump's latest push to be more inclusive, encapsulated by "I'm with You." But at the same time, he continues to advocate policies that alienate large groups of voters.
In a Washington Post-ABC News poll released last week 70 percent of Americans had an unfavorable view of Trump, including 77 percent of women and 88 percent of nonwhite voters. Clinton received a 55 percent negative rating the in the poll.
What remains to be seen is whether centrist swing voters will embrace "I'm With You" as a genuine call for unity or reject it as empty posturing.
"There is no such thing as having enough Donald Trump Twitter for me," said Democratic strategist Jim Manley, who supports Clinton. "Every tweet is guaranteed to offend someone no matter what the hashtag says."