PHILADELPHIA — In a fresh indication of the challenges Democrats face unifying the party in the fall, the campaign of Bernie Sanders sent out a fundraising solicitation Tuesday that included a picture of a smiling Hillary Clinton standing next to Republican front-runner Donald Trump at his wedding.

“Over the past few days, Hillary Clinton’s campaign and some of its top supporters have launched an odd new line of attack against people like you who stand with Bernie,” said the email signed by Jeff Weaver, Sanders’s campaign manager. “They are saying that by continuing to campaign and fight for every vote, for every delegate, that we are helping Donald Trump.”

“They’ve used language reserved for traitors to our country, saying we are ‘giving aid and comfort’ to Trump,” Weaver continued. “They are emailing supporters with the subject line ‘What Trump loves about Bernie.’ Let me be clear, there is one candidate in this Democratic primary who Donald Trump said would make a ‘great president,’ and it’s not Bernie Sanders.”

The jab at Clinton came on a day when voters in five states — Pennsylvania, Maryland, Connecticut, Rhode Island and Delaware — were taking part in Democratic primaries that could render Sanders’s already narrow path to the nomination virtually nonexistent. Polls in recent days have showed Clinton leading in most contests, including the two with the most delegates at stake, Pennsylvania and Maryland.

The photo that was part Sanders’s fundraising solicitation shows Clinton and Trump and their respective spouses, Bill Clinton and Melania Trump, sharing what appeared to be a light moment at the wedding reception.

In recent days, Sanders has repeatedly said in interviews that he intends to stay in the race through the final nominating contests in June and that it will largely be up to Clinton to earn the support of his voters if she is the party’s nominee.

The senator from Vermont has suggested tha it would be helpful if Clinton would adopt some of his positions on issues such as universal health care. Clinton has instead pointed to her unconditional support of President Obama in 2008 after he defeated her for the Democratic nomination.

“We are going to keep fighting hard for every vote and for every delegate,” Weaver said in the email. “Not just because that’s the only way to win this nomination, but because we believe the power of our progressive agenda would energize the kind of high voter turnout needed to stop Trump.”