Most of all, they say, they worry that Ms. Collins, despite her good intentions, is getting duped by Republican leaders eager to notch their first major legislative victory since assuming control of Congress and the White House.
The event was closed to the news media, but streamed live over Facebook.
“You have to protect us before you vote yes,” said Mr. Barkan said to Ms. Collins during the meeting. (Last week, Mr. Barkan confronted Senator Jeff Flake, Republican of Arizona, about the tax bill on an airplane.) “They’re lying to you.”
Ms. Collins remained respectful and strained to convince the room of about a dozen skeptics that the promises that had been made to her were ironclad.
She defended her decision in the face of the group’s challenges that previous Republican promises for the tax bill had been broken, including a commitment to not add to the deficit and to not benefit the rich, and that written agreements are not law.
“I do not believe that I’ve given up leverage,” Ms. Collins said. “I’ve used my leverage to negotiate agreements that are promises to me.”
She added, “I’m sorry that you don’t believe in the agreements.”
The deals that Ms. Collins struck that extend beyond the tax bill have been the subject of much speculation and doubt in Washington, because they would require the backing of House Republicans and some Democrats.
But around the halls of the Capitol on Wednesday, Ms. Collins appeared to be increasingly comfortable with voting for the tax cuts.
She said that she had a positive discussion on Tuesday with Vice President Mike Pence about her health care concerns and that she was generally supportive of the shape that the final tax bill was taking.
Ms. Collins also was not biting on Democrats calls to delay the vote. She rejected the suggestions that the tax vote should be put off so that Doug Jones, the Alabama Democrat who was elected to the Senate on Tuesday night after a special election, could be seated first and cast a vote.
“I see no need to wait for Doug Jones to become a senator,” Ms. Collins said. “We vote all the time in lame-duck sessions with retired and defeated members casting votes.”
After her meeting at her office, it did not appear that Ms. Collins was ready to change her vote, or that she had been brought to tears.
But Ms. Collins said she appreciated the chance to hear her critics out and explain herself.
Asked whether she was moved by the meeting, she replied, simply, “Of course.”