How did Susan Collins go from being one of the most popular senators in the country to one of the most endangered?

She blames the flood of negative ads put up by her Democratic opponent, Sara Gideon, what Collins calls “the unfortunate lies that have been so prevalent on television for the past two years.”

But the slide in her popularity started three years ago, long before the ads began filling the airwaves. It even predates her 2018 speech and vote in support of Supreme Court Justice Brett Kavanaugh

Collins enjoyed a 62 percent approval rating among Mainers in October 2017, according to polling by Morning Consult. But just three months later, her approval rating in the same poll had fallen to 53 percent, and it’s been on a downward trajectory ever since.

What happened between October and January? It was her support for the Tax Cuts and Jobs Act – better known as the Trump tax cuts – that started to change the way Mainers thought about their senior senator.

In the first year of the Trump administration, Republicans controlled the House and Senate, and the only thing that could get in their way was principled opposition from Republican senators like Collins, who had just joined with two colleagues to stop the runaway freight train of Obamacare repeal.

But when the Republican Party came up with a plan for a massive $1.5 trillion tax giveaway to the rich that would bust a hole in the deficit they had claimed to care about when Barack Obama was president, Collins did not stand in their way. Instead, she took a leading role in championing the bill.

She started by taking a trip to Maine with Ivanka Trump, who gave the impression that the bill was really a vehicle to help working families pay for child care, and not windfalls for wealthy families like her own.

Collins even went on “Meet the Press” later that fall to promote the unsupportable claim that tax cuts pay for themselves.

“Economic growth produces more revenue and that will help offset this tax cut and actually lower the debt,” Collins explained to NBC’s Chuck Todd, who shook his head in amazement.

Collins said in negotiations that she had made the bill better for the middle class, allowing bigger deductions for medical expenses, property taxes and child care tax credits. Additionally, she said, “I got a commitment that we’re going to pass two bills” that would reduce health insurance premiums.

Collins never got her two bills, but what about the other promises of the 2017 tax legislation?

Cutting corporate taxes was supposed to increase investment in plants and equipment and boost worker pay. It was also supposed generate more revenue to run the government because it would stimulate growth.

It failed on all three counts.

Corporations invested some of their windfall, but according a study by the Federal Reserve Bank of Cleveland, they did so at a slower rate after the tax cut than in the years leading up to it. Companies spent even more of the money they got from the government to buy buy back some of their own stock, jacking up the price of the remaining shares. That helps people who still own the stock, but it does nothing to improve productivity or create jobs.

The tax bill was supposed to generate bigger paychecks for workers, but that didn’t really happen, either. There was some wage growth in 2018, but adjusted for inflation wages grew at a slower rate than the economy as a whole.

Despite provisions for middle class families, the benefits went exactly where the critics said they would. Taxpayers with incomes between $50,000 and $100,000 received refunds that were 2.5 percent higher. Those with incomes over $200,000 got 45 percent more back.

And did the tax cuts pay for themselves? Sorry.

The actual amount of tax revenue collected in 2018 was significantly lower than what the Congressional Budget Office had projected before the tax cut was signed into law, according to a study by the Brookings Institute. “Given that the economy grew in 2018, the data imply that the 2017 tax cut substantially reduced revenues,” Brookings found.

All of this was predictable, which is why Collins’ support for the bill has been a turning point. After 40 years in which supply side economics has proven to be good for the rich and bad for everyone else, Collins doubled down on trickle down.

Now she’s down in most of the polls, and it’s not just because she’s a victim of negative campaigning, or because Donald Trump is unpopular.

The slide has been going for a while, and it started with the 2017 tax bill.