Instead, the president planned to make the trip to the northwestern Pennsylvania city of Erie without her, still aiming to defend his narrow win in the state four years ago.
"Mrs. Trump continues to feel better every day following her recovery from COVID-19, but with a lingering cough, and out of an abundance of caution, she will not be traveling today," the first lady's chief of staff and spokeswoman, Stephanie Grisham, said in a statement.
The cancellation came as a surprise, after the president said just Tuesday morning in an interview on "Fox and Friends" that the first lady was "doing great" following her battle with the coronavirus.
Last week, Melania Trump wrote about her personal experience with the virus on the White House website and said that she experienced headaches, fatigue, body aches, and a cough during her illness and described her symptoms as “[hitting] me all at once and it seemed to be a roller coaster of symptoms in the days after.”
Her absence is a setback for her husband, who is desperate to persuade women in the critical battleground state to support his reelection bid. The last time Trump was in the state, a week ago, he literally pleaded with "suburban women" to like him.
“Suburban women, they should like me more than anybody here tonight because I ended deregulation that destroyed your neighborhood” Trump said, referring to an Obama-era regulation that sought to combat segregation in housing but was halted by his administration.
"So can I ask you to do me a favor? Suburban women, will you please like me? I saved your damn neighborhood,” the president said at an Oct. 13 rally in Johnstown, Pennsylvania.
In 2016, Melania Trump proved a valuable closer to the president in Pennsylvania, holding her only solo event of the campaign in the state and adding a softer side to her husband’s brash and divisive style of politics, professing his "deep love and respect for this country, and all of its people."
Aside from addressing the Republican National Convention from the Rose Garden in August, the first lady has made no public appearances to boost her president’s campaign this year.
Back in March, the first lady had been scheduled to hold a fundraiser on her husband’s behalf, but that appearance was cancelled. Her office at the time cited an unspecified “scheduling conflict.” The cancellation came amid a rise in coronavirus cases at the time.
Her last appearance on the campaign trail was over a year ago, when she joined her husband on stage in Orlando at his reelection kickoff in June 2019.
Trump's Erie rally was part of an attempt to defend his narrow win in Pennsylvania in 2016; fewer than 2,000 votes separated him from his Democratic opponent, former Secretary of State Hillary Clinton. Four years earlier, then-President Barack Obama had won the state by nearly 17%.
Recent polls have shown former Vice President Joe Biden with a clear lead in Pennsylvania. In a Quinnipiac poll released Oct. 7, the Democratic nominee led by 13% among likely voters. A poll from Monmouth University published the day prior showed a similar lead, with Biden up 12% among registered voters.
FiveThirtyEight's Pennsylvania polling average shows Biden leading by 6.4%.
One part of the state that contributed to Trump's win was its northwestern corner around Erie. Emblematic of the white-working class communities in the midwest that traditionally voted Democratic, the area flipped for Trump in 2016 -- albeit narrowly, by less than one percentage point.
"I'm going to Erie, Pennsylvania today, which is -- which I love," Trump said in his interview with Fox News Tuesday morning. "You know, that was always a Dem stronghold and then I came along and we shook it up."
Erie County is one of just three of the state's 67 counties that voted twice for Obama and then for Trump -- so-called "pivot counties." Obama won the county by more nearly 20% in 2008, and by more than 15% in 2012.
In a Monday phone call, Trump told his campaign staff that "we're working hard on Pennsylvania."
"I think we'd win Pennsylvania," Trump said. "I think it's a sleeper like it was last time."
ABC News' Benjamin Siegel and Quinn Scanlan contributed reporting.