Sen. Cory Booker of New Jersey is circulating an online petition calling on President Trump to resign over allegations of sexual harassment.
Mr. Booker is among a growing list of possible contenders for the Democratic presidential nomination in 2020 who have demanded Mr. Trump leave the White House.
"The number of women who have brought sexual harassment, misconduct, or assault allegations against him has grown to 20 individuals - and I believe them. We all should believe them," Mr. Booker said in the email. "We must hold members of both parties to the same standard, and that standard should apply all the way to the top.
"President Trump must be accountable for his actions," he said. "Please join me in calling on Mr. Trump to resign as President of the United States."
Mr. Trump has denied the accusations leveled against him.
Earlier this month, Mr. Booker called on fellow Democratic Sen. Al Franken of Minnesota to step down in the face of mounting sexual harassment allegations.
"Let's show President Trump that all of our leaders must be held accountable and that we are not backing down," Mr. Booker said.
The list of senators on Capitol Hill urging Mr. Trump to step down includes three more possible 2020 contenders: Democrats Kirsten Gillibrand of New York and Kamala Harris of California, as well as Bernard Sanders, Vermont independent.
Billionaire environmentalist Tom Steyer, meanwhile, has invested tens of millions of dollars into television ads calling on Mr. Trump to be impeached, stoking speculation that the environmental activist also might launch a presidential bid.