Washington (CNN) A growing number of senators have called for President Donald Trump's resignation in the face of such sexual harassment and assault accusations.

Several of the senators cited the resignation of Democratic Sen. Al Franken amid sexual harassment allegations as an increased impetus for Trump to do the same.

At least 15 women have come forward with a wide range of accusations against Trump, ranging from sexual harassment and sexual assault to lewd behavior around women. Of the women, 13 say Trump attacked them directly and two others say they witnessed behavior that made them uncomfortable. All the alleged incidents took place prior to Trump's assuming the presidency.

Trump has denied the allegations, most recently dismissing them as "fake news" in a Tuesday morning tweet.

These are the senators calling on the President to resign:

Sen. Kirsten Gillibrand, D-New York

On Monday, the Gillibrand told CNN that Trump should step down.

"President Trump has committed assault, according to these women, and those are very credible allegations of misconduct and criminal activity, and he should be fully investigated and he should resign," Gillibrand told CNN's Christiane Amanpour in an exclusive interview.

"These allegations are credible; they are numerous," said Gillibrand, a leading voice in Congress for combating sexual assault in the military. "I've heard these women's testimony, and many of them are heartbreaking."

The New York Democrat said that Congress "should have appropriate investigations of his behavior and hold him accountable" if Trump does not step down.

Sen. Jeff Merkley, D-Oregon

The Oregon Democrat first called for Trump to step down in an email fundraising pitch Friday.

"I want to be absolutely clear. Donald Trump should resign the presidency," he said in the email, adding that he believes the women who have accused the President.

"This is not about politics. This is not about policy. I disagree with him on many things, but this is not about that," Merkley said in the email.

On Tuesday, Merkley said that there should be a congressional hearing into Trump's behavior if he does not resign.

"I would like to see there be a hearing on Capitol Hill," he told CNN's Alisyn Camerota on "New Day." "If there's not, if Republicans are unwilling to have this conversation about the President, then we should hold a shadow hearing with the Democratic caucus."

Merkley also dismissed the notion that the calls for resignation were a Democratic publicity stunt, pointing to his party's collective action on Franken.

"It's fundamental accountability. that's what it boils down to," he said. "There were over 40 senators who said it was the right thing for senator franken to step down. But if you believe that that's the case, wouldn't you also believe that it's the right thing for the president to step down, who has far more serious set of stories?"

Sen. Ron Wyden, D-Oregon

Wyden joined Merkley, his fellow Oregonian, on Monday in issuing a call for Trump to resign.

These women are right. If @realDonaldTrump won't resign, Congress must investigate allegations by many, many women that he sexually assaulted and harassed them. No one is above the law. https://t.co/ySP9DVUgLP — Ron Wyden (@RonWyden) December 11, 2017

"If @realDonaldTrump won't resign, Congress must investigate allegations by many, many women that he sexually assaulted and harassed them. No one is above the law," he wrote.

Sen. Cory Booker, D-New Jersey

After a campaign event for Democratic Alabama Senate candidate Doug Jones on Saturday night, Booker told Vice News that he believes Trump should resign.

"I just watched Sen. Al Franken do the honorable thing and resign from his office. My question is, why isn't Donald Trump doing the same thing -- who has more serious allegations against him, with more women who have come forward? The fact pattern on him is far more damning than the fact pattern on Al Franken," the New Jersey Democrat said.

Sen. Mazie Hirono, D-Hawaii

Hirono took to Twitter on Tuesday to defend Gillibrand and echo calls for Trump to resign.

".@realDonaldTrump is a misogynist, compulsive liar, and admitted sexual predator. Attacks on Kirsten are the latest example that no one is safe from this bully. He must resign," she wrote.

.@realDonaldTrump is a misogynist, compulsive liar, and admitted sexual predator. Attacks on Kirsten are the latest example that no one is safe from this bully. He must resign. https://t.co/7lNI23K7ib — Senator Mazie Hirono (@maziehirono) December 12, 2017

Sen. Bernie Sanders, I-Vermont

On Wednesday evening, Sanders joined his Democratic colleagues to call for the President to resign over the allegations against him.

"Yes, I do think he should resign," Sanders said on CNN's "Anderson Cooper 360."

"You've got a president who has been accused by many, many women of harassment, to say the least. This is a guy who was on a tape seen by everybody in America essentially bragging about his sexual assault of women," the Vermont independent said.

However, he did not express optimism that Trump would step down.

"Do I think under those considerations of Al Franken resigning, do I think the President should resign? I do. Do I think he will? I don't," he said.