She added, "The horrific tragedy in Pittsburgh is not a political event."
The trip was announced amid mounting opposition to a presidential stop-in by protesters, politicians and at least one of the victims' families.
On Monday, Pittsburgh Mayor Bill Peduto admonished the White House not to visit the city "while we are burying the dead."
"Our attention and our focus is going to be on them, and we don't have public safety that we can take away from what is needed in order to do both," Peduto, a Democrat, told reporters just before Sanders announced the trip.
Peduto also said Trump should ask the families of the victims whether they want him to visit — "That's not my call to make," he said. Peduto told The Washington Post that he declined an invitation to appear with the president.
The Post reported Tuesday morning that the family of 71-year-old Daniel Stein, who was killed in the massacre, shunned Trump because of his comments in the immediate aftermath of the shooting. The president had suggested that armed guards in the synagogue may have prevented the tragedy: "If they had protection inside, the results would have been far better," Trump said.
Stein's nephew, Stephen Halle, told the newspaper that "Everybody feels that [Trump's remarks] were inappropriate."
More than 1,200 people have RSVP'd to a Facebook event organizing a demonstration against Trump's visit Tuesday, and some community leaders have also pushed back on the trip.
But Rabbi Jeffrey Myers, who was leading a service at Tree of Life when the shooter attacked, told CNN: "The president of the United States is always welcome. I am a citizen. He is my president. He is certainly welcome."
The White House had invited at least four top lawmakers to come to the city, sources familiar with the situation told CNBC. But all four declined to make the trip, the sources said.
"The leader was unable to attend today," said Stephanie Penn, spokeswoman for Senate Majority Leader Mitch McConnell, R-Ky., after saying the invitation was "not declined." McConnell will instead travel within his state to Brandenburg and Rough River Lake, and will discuss a water infrastructure law signed by Trump in October.
"We weren't able to make it on the short notice," House Speaker Paul Ryan's spokeswoman, AshLee Strong, told CNBC. The White House's announcement came about a day before Air Force One was scheduled to arrive at Pittsburgh International Airport at 3:45 p.m. ET.
In a statement Saturday, Ryan said: "This is a time to mourn and heal, but also to reaffirm that we will not tolerate this bigotry. Pittsburgh and the Jewish community have been rocked today, but they should know that Americans stand firmly with them."
The top Democrats in Congress — House Minority Leader Nancy Pelosi, D-Calif., and Senate Minority Leader Chuck Schumer, D-N.Y. — also declined invitations, two sources told CNBC. Schumer declined the invitation due to a scheduling conflict, one of the sources said.
The office of Pennsylvania's Democratic governor, Tom Wolf, did not respond to CNBC's inquiries about whether he had received an invitation from the White House. The Trump administration did not respond to CNBC's outreach.
In an interview with Fox News that aired Monday evening, Trump said, "I'm just going to pay my respects."
The president added that he is "also going to the hospital to see the officers and some of the people that were so badly hurt. So, and I really look forward to going. I would have done it even sooner, but I didn't want to disrupt anymore than they already had disruption."
Correction: This story was revised to correct that Schumer is the Senate minority leader.