Philadelphia (CNN) A jovial President Barack Obama lit into Donald Trump Tuesday, using his first solo campaign appearance to openly question the nominee's fitness for the presidency.

Obama didn't directly address Hillary Clinton's bout with pneumonia, which has kept her housebound as the campaign enters its final stretch. But he did issue a warning for anyone who's questioning the Democratic candidate's ability to perform the job.

"You want to debate who's more fit to be president? One candidate has traveled to more countries than any other secretary of state has. Has more qualifications than any candidate in history. And the other who isn't fit in any way shape or form to represent this country abroad or to be its commander in chief," Obama said at roaring campaign rally in Philadelphia.

He dismissed questions about Clinton's transparency, which arose after she kept quiet her pneumonia diagnosis for several days. Instead, he ripped into Trump's decision to withhold his tax returns, a historic break from precedent.

"You want to debate transparency? You've got one candidate in this race who's released decades' worth of her tax returns. The other candidate is the first in decades who refuses to release any at all," Obama said.

For Obama, Tuesday's event was a return to the rollicking campaign events that thrust him into the White House eight years ago, and won him reelection in 2012. He exclaimed as he took the stage here it was "good to be back on the campaign trail," a sentiment that was returned by loud cheers by the crowd of hundreds.

"Can I just say I am really into electing Hillary Clinton?" Obama told the crowd, which enthusiastically greeted him. "This is not me just going through the motions here. I really, really, really want to elect Hillary Clinton."

Obama is just one of a cavalry of top White House Democrats -- also including First Lady Michelle Obama and Vice President Joe Biden -- who are arguing Clinton's case this week as the Democratic nominee herself is convalescing from pneumonia and absent from the trail.

The timing is opportune for a campaign eager both to spotlight its most popular surrogates and to move past a rocky patch. While Obama's Philadelphia stop was planned well ahead of Clinton's declaration that half of her rival's supporters were "deplorable" and new worries about her transparency, the campaign hopes the President's rally can at least provide a new storyline.

But even an appearance from Obama -- whose approval rating reached a nearly eight-year high of 58% in an ABC/Washington Post poll Monday -- won't necessarily cure all of Clinton's woes as the campaign enters its busiest stretch. The White House Monday said Obama would not be relegated to "damage control" for Clinton's stumbles, and he declined to address either the "deplorable" dust-up or her illness directly.

Instead, Obama leaned hard into his criticisms of Trump, using his stature as commander in chief to disqualify the Republican candidate.

"You've got the Donald, who just last week went on Russian television to talk down our military and curry favor with Vladimir Putin," Obama said, referring to an interview appearance on RT. "He loves this guy! Think about what's happened in the Republican Party," Obama said. "They used to be opposed to Russia and authoritarianism and fighting for freedom. Now their nominee is out there praising a guy, saying he's a strong leader, because he invades smaller countries, jails his opponents."

"This isn't Abraham Lincoln's Republican Party," Obama added later. "This is a dark, pessimistic vision of a country where we turn against one another, where we turn against the world. They aren't offering serious solutions, they are fanning resentment, and hate. That is not the America I know."

Michelle Obama is also stumping for Clinton this week. The first lady makes her first campaign appearance for the Democratic nominee on Friday, promoting voter registration in Northern Virginia. Loathe to engage directly in bitter partisan politics, the first lady is more likely to spell out a more affirmative case for Clinton, according to aides.

Biden, who talked up Clinton during a stop Monday in Charlotte, was more candid in his assessments, suggesting the candidate gets a "bum rap" when he was asked about her remarks casting some of Trump's supporters as deplorable.

"For every time she will say something where she says, 'Well, maybe I should have said something different,' think if they held Trump to that standard," Biden said. "He'd be in trouble. He is in trouble."

Obama's busy schedule a factor

Even as Clinton is increasingly relying on Obama to carry her message to young and minority voters, the demands on the President's time have largely forestalled an aggressive campaign schedule thus far.

Tuesday's event in Philadelphia is only Obama's second campaign stop for Clinton, after a joint appearance in Charlotte in July. Since then, Obama has helped raised money from Democratic donors, including during his vacation on Martha's Vineyard last month, but he hasn't headlined another rally until now.

White House officials point to a largely inflexible schedule of presidential commitments this month as a barrier to more frequent campaigning. While past presidents have faced similar obligations in the waning days of their tenures, Obama is more popular -- and thus in higher demand as a campaigner than his most recent predecessors.

In August, Obama's aides made a day-by-day assessment of the President's commitments until election day, discovering few moments in September that would allow for rallies in key battleground states on behalf of Clinton.

Obama on Friday concluded a week-long swing through Asia, with stops on the front end in Nevada, Hawaii and Midway Island meant to burnish his environmental legacy.

Even Tuesday's rally was restricted to a relatively close-by location. Obama had business at the White House Monday evening when he met with congressional leaders, and is due to meet Burmese State Counselor Aung San Suu Kyi in the Oval Office on Wednesday.

Obama's four-day obligation at the United Nations General Assembly next week makes adding a campaign stop difficult before the end of the month. And while the campaign may arise implicitly during Obama's final address to the gathering of world leaders -- he's expected to recap eight years of foreign policy, providing a contrast to Trump's proposals -- it's hardly the setting for a fiery political throw-down.

Even in October, when the race will enter its frenzied sprint, the demands of the presidency mean an all-out, every-day-on-the-trail presence for Obama is unrealistic. Already the President's schedule is filling up. The White House announced Monday that Italian Prime Minister Matteo Renzi would visit on October 18, a day-long affair that will stretch into a late State Dinner. A government funding battle also seems likely to occupy the President's time this month.

Many of the states that officials say Obama will target -- including Florida, North Carolina, Iowa, Pennsylvania and Michigan -- are easily accessible in a single day-trip on Air Force One, but would leave few moments for other business at the White House.

Given Obama's sway among young and minority voters -- populations historically difficult to get to the polls -- there is pressure for the President to hit many states before their voting registration deadline passes. In Florida, Ohio, Michigan, and Pennsylvania, that means showing up before October 11.

"I think what is clear is that the President does have a lot of influence over a large number of voters that haven't previously been regularly engaged in politics," Earnest said Monday, adding the Clinton campaign is "hoping that the President will be helpful in making the case on their behalf to motivate voters to get registered and to participate on Election Day."