The problem is, House Republicans largely oppose the health-care bills. And while Collins anticipates that the commitments will be included in must-pass spending legislation to keep the government open, the tax package is scheduled for a final vote next week, before the spending measure.
That means Collins will have to cast her vote on the tax bill without knowing for certain that commitments made to her will be honored, leading critics to say she's getting played for a fool.
If she prevails, Collins will have been responsible for the passage of significant legislation that could help make insurance coverage more affordable for tens of thousands of Americans.
And if not?
"I'm counting on the administration to make sure that does not happen," Collins said in an interview. "I would consider it a very serious breach of a promise to me."
"And," Collins added with a laugh, "they don't want to do that."
Indeed, with the fourth-term senator poised to play an even more pivotal role in the Senate next year, she might be among the last people President Trump or Majority Leader Mitch McConnell (R-Ky.) wants to alienate.
One of the few remaining moderate Republicans in the Senate, the 65-year-old Collins has repeatedly used her vote to sway outcomes and shape the debate on issues from nominations to health care to the environment. She passed up a run for Maine governor earlier this year even though she would have been the instant front-runner, saying she believed her influence was best wielded from the Senate floor.
Because of the upset win by Democrat Doug Jones in a special election for Senate in Alabama on Tuesday, the GOP's already slim 52-to-48 Senate majority will dwindle to 51 to 49 next year. That will only increase Collins's influence, giving the diligent lawmaker a pivotal role in every legislative fight for the remainder of Trump's first term and beyond.
"The closer your margins between majority and minority, there is a greater role for those who are very deliberate in how they're moving forward, who are working with both sides of the aisle in a very clear and demonstrated way," said Sen. Lisa Murkowski (R-Alaska), Collins's close friend who sometimes joins with her to oppose fellow Republicans. "And I think that Sen. Collins does exactly that."
Collins, Murkowski and Sen. John McCain (R-Ariz.) were the three Republicans who killed the GOP's efforts to repeal the Affordable Care Act earlier this year, driving a stake through seven years of their party's promises to undo former president Barack Obama's signature legislative achievement.
When she returned home to Maine after casting that pivotal vote, Collins was greeted as a hero. Passengers in the Bangor airport broke into applause when she stepped off her plane, a scene Collins later described as "heartwarming and affirming."
The reaction at home to her stance on the tax legislation has been quite different. The tax bill, which increases the deficit while delivering huge benefits to corporations and the wealthy, polls poorly and has sparked repeated protests against Collins. Students have staged sit-ins while religious leaders were arrested after occupying one of her offices.
After Collins's votes against the repeal of the ACA, also known as Obamacare, some Maine residents have struggled to understand why she would support a tax bill that slashes corporate rates while repealing the ACA's mandate for most Americans to carry health insurance or pay fines. The individual mandate repeal is expected to result in 13 million more uninsured Americans, and health-care advocates say that even if they do end up becoming law, the health bills Collins supports will not make up for that.
"It's the last vote that counts. You can say 'no' 53 times in a row but if the 54th is the 'yes' that puts the thing over the line, that's going to be your legacy," said Steve Butterfield, policy director of Consumers for Affordable Health Care in Augusta, Maine.
In this case, Butterfield added, Collins's legacy would be millions of Americans losing their health insurance.
"It's absolutely unbelievable," he said.
Collins counters that she has never liked the individual mandate, because it forces consumers to buy a product they may not want, and the fines on abstainers fall disproportionately on people who make less than $50,000 a year. She draws a distinction between the individual mandate repeal in the tax legislation and the earlier GOP health bills that she opposed, which in addition to the mandate repeal also kicked millions of people off Medicaid, which serves the poor, among other changes.
"There's a big difference between forcing someone to buy insurance that they deem unaffordable versus taking away from people insurance that they already have, need and want," Collins said, "which is what the health-care bills last summer would have done."
Nonetheless, Collins opposed including the mandate repeal in the tax legislation, saying the issues should be addressed separately. Once Republican leaders decided to include it, partly because it raised nearly $340 billion in revenue they could use to lower taxes for corporations and make other changes, Collins began arguing that other health-care legislation would be needed to stabilize insurance markets and keep premiums from spiking. The Congressional Budget Office estimated that the individual mandate repeal would lead to premiums rising 10 percent in the individual insurance market.
One of the bills Collins supports, authored by Sens. Lamar Alexander (R-Tenn.) and Patty Murray (D-Wash.), restores "cost-sharing" reduction payments that help insurers reduce costs for lower-income Americans under the ACA. Congressional Republicans had challenged the federal payments in court during the Obama administration, and Trump decided in October to stop making them.
The other bill is one Collins co-authored with Sen. Bill Nelson (D-Fla.) to create a $10 billion "reinsurance" program that states could draw on over two years to set up high-risk pools that lower costs for patients who are particularly hard to insure.
In addition to pushing for the health-care bills, Collins negotiated several other changes to the tax legislation, including preserving taxpayers' ability to deduct medical expenses and allowing them to deduct up to $10,000 in state and local taxes from their federal tax bill. Those changes were included in the tax legislation itself, instead of relying on the promise of future action by Congress.
Collins's moves on the health bills have been met with skepticism, if not downright ridicule, from critics who note that she has already backed off her call for the health legislation to pass before the tax bill. Instead, she now says the health bills must pass by the end of this year, pointing to a written pledge from McConnell and to verbal commitments from administration officials including Vice President Pence and Trump.
Some Democrats argue that those promises will not amount to much.
"How any senator — much less one who has served as long as Sen. Collins has — ever agreed to such a deal is beyond me," said Jim Manley, a Democratic consultant and former high-ranking Senate aide. "The promises she extracted were never, ever going to be binding on rank-and-file House Republicans, and now she has nothing to show for all of this. She got rolled big time."
Whether that's the case remains to be seen. A must-pass year-end spending bill presents the only realistic vehicle to carry the health bills, but it's uncertain whether the final maneuvers on that legislation will play out as Collins would wish. Democratic votes will be needed in the Senate, but Democrats have not committed to supporting her priorities, nor have House Republicans, many of whom oppose the health bills and Collins's attempts to push them into law over their objections.
"You've got individual senators that think, in my view arrogantly, that they are entitled to disproportionate representation of the country," said Rep. Dave Brat (R-Va.), a member of the conservative Freedom Caucus. "I mean, the American people ought to be ticked off."
In the end, Collins may have little recourse if the bills she's touting don't become law, other than making known her displeasure with the Trump administration and her party's leadership in Congress.
Yet given the pivotal position she occupies, that may just be enough to push GOP leaders from the president on down to do everything they can to keep her happy.
Senate Republicans seem to know that.
"It's good," said Sen. Pat Roberts (R-Kan.), "to have Susan in the tent."