A constituent told her she lacked courage. U.S. Sen. Susan Collins thought that was rude.

That, in a nutshell, is the story behind a full-page ad that appeared in last weekend’s Maine Sunday Telegram, signed and paid for by Erik Mercer of Portland.

“Senator Collins, you said I was rude,” began the ad, which cost Mercer just over $7,200 and went viral on Twitter and other social media platforms.

Here’s what happened:

Mercer was waiting for a flight from Washington, D.C., to Portland on Oct. 9 when he spotted Maine’s Republican senator seated in the same airport gate area, talking on her phone. As she finished the call, he approached her and asked if they could chat. Collins agreed and invited him to sit down.

Mercer, a social worker who has a psychotherapy practice in Portland and also works with prisoners on death row all over the country, wanted to get answers from Collins to several questions that had been dogging him for months.

Such as: What should he tell his young daughter about a president who speaks of women, immigrants and minorities in language that would not be tolerated in her classroom?

Collins, according to Mercer, responded by lamenting the “lack of civility” in the nation’s capital.

“I hear you,” Mercer later recalled telling her. “But I think we can all agree that the main offender is the president.”

And?

“She wouldn’t even talk about Trump by name,” Mercer said in an interview Wednesday. “She just talked around the whole thing. It was a non-answer.”

Mercer pressed ahead with questions about Collins’ vote to confirm Brett Kavanaugh to the U.S. Supreme Court, about whether she considered it a crime for someone to solicit foreign interference in a U.S. election, about her views of the current impeachment proceedings against Trump.

“And again, she just kept saying, ‘Well, we have a difference of opinion,’” Mercer recalled. “And I said to her, ‘I suspect we do have a difference of opinion, but you still haven’t told me what yours is!’”

At one point, Mercer admits, he told Collins, “I think you lack courage.” But once it became clear they were getting nowhere, Mercer went his own way and called a friend on his cellphone to vent – although again, he said, he never raised his voice during that conversation.

Then the flight began boarding. Mercer found himself two people behind Collins on the jet bridge when he heard her tell another passenger that a constituent “had just been very rude” to her.

That did it. Upon arriving in Portland, Mercer decided to take out the ad laying out what had happened for all of Maine to see. In it, he repeated what he says he told the senator on the jet bridge upon hearing her call him “rude.”

“It was not my intention to be rude,” he told Collins in the ad. “It was my intention to have a thoughtful discussion across ideological lines. It was my intention to do the work of democracy, but you were unwilling to participate in that.”

With that, the battle lines were drawn. In two telephone calls Wednesday, Collins spokeswoman Annie Clark took aim squarely at Mercer.

She sent links to the Federal Elections Commission website showing that since 2004 Mercer has made 426 donations totaling $16,108 to “Democrats and far-left activist groups.”

“That’s democracy,” Mercer responded, noting that the vast majority of the donations, which range from $2.50 to $200, were made over the past 15 years through automatic monthly payments to such organizations as MoveOn.org, ActBlue and the Democratic Senatorial Campaign Committee.

As for the “rude” remark that set the whole newspaper ad in motion, Clark said the senator recalls a person in the boarding line commenting to her that “that person (Mercer) was rude to you.” But Collins “doesn’t recall” whether she used the word herself, Clark said.

Even when Mercer insists that the word “rude” is burned on his brain and he’ll never forget hearing Collins say it?

“Would I disagree with that characterization?” Clark replied. “Probably not.”

How clarifying. Maybe Collins said it, but if she did, she doesn’t recall saying it.

“She did,” Mercer said. “I recall.”

Clark claims that Mercer, in addition to telling Collins to her face that she has “no courage,” also told the senator she has “no principles.”

Mercer said he never made any mention of Collins’s principles – or lack thereof.

Finally, Clark said Collins described Mercer’s demeanor throughout the encounter as “aggressive, confrontational and sanctimonious.” Over the course of almost an hour, Clark repeated the words at least three times.

Countered Mercer, “That is a flat-out lie. I’m really stunned. Maybe I shouldn’t be, but that is an absolute, 100 percent lie.”

Enter the eyewitness. Barbara Alexander of Winthrop was on the same flight, observed Mercer’s conversation with Collins in the gate area and took to Facebook on Sunday to back him up.

“I was there and saw this entire interaction,” Alexander wrote. “(Mercer) was polite and respectful. He asked (Collins) if it was OK to talk with her. There was no confrontation. She just refused to engage and answer his legitimate questions. He disengaged without any rancor.”

Alexander, a registered Democrat and vice chair of the Kennebec County Democratic Committee, repeated in an interview Wednesday that Mercer, whom she said she’d never met before they ended up as seatmates on the plane, was nothing but a gentleman throughout his brief tête-à-tête with Collins.

“The conversation was entirely quiet,” Alexander said. “I mean, I never heard an argument or loud voices or anything of that kind.”

Back to Clark, who also sent a written statement Wednesday lamenting the fact that Collins now requires security wherever she goes both in Washington, D.C., and Maine:

“Over the past two years Senator Collins has endured death threats, threatening mailings, been confronted by people at her home late at night, been harassed in airports, at stores, when eating out, and in parking lots. She has protesters regularly at home, at her offices, and at events. She is ridiculed regularly online by people who mock her intellect, integrity, and physical characteristics. And through all that, for 23 straight years, she has never missed a day of work.”

(Maybe it’s just me, but if I was experiencing all of those things, I’d start wondering what the heck I’m doing to tick so many people off.)

Clark added, “Senator Collins doesn’t think she called Mr. Mercer rude, but she definitely did think that he was.”

Not exactly an olive branch. Nor is it the end of the story.

Since last weekend’s ad went into cyber orbit, Mercer has received numerous donations to cover its steep cost – and then some.

“There was an outpouring of support for my message,” he said Thursday. “People wanted to keep this going. And so, I decided to put in another one.”

Mercer’s second full-pager will run Sunday. It will include a growing list of folks from far and wide who want their names to appear alongside Mercer’s.

Like the man said, that’s democracy.