For those elected to high office, there’s often a moment that defines one to the nation, and creates a legacy. For Sen. Susan Collins, that moment may be at hand.

Collins was elected as a moderate Republican in 1996, at a time when moderate Republicans were common in New England, and not rare elsewhere. Twenty years later, the landscape has changed almost beyond recognition.

Collins is now the last Republican senator from the six New England states, after Kelly Ayotte’s defeat in New Hampshire, and the only Republican, nationally, with any claim to the “moderate” designation.

It hasn’t been easy. As her political party, and the entire political spectrum, veered far to the right, Collins has struggled just to keep afloat in her caucus. Her former colleague, Olympia Snowe, left the Senate in 2012 at least in part because her views on the budget, health care and other vital issues were no longer welcome in the GOP caucus.

To her constituents, Collins is doing better than most. Surveys show her with a glowing approval rating, and she’s been re-elected three times by wide margins, achieving a rare distinction; only three other Mainers have been popularly elected to four U.S. Senate terms, the others being Frederick Hale (1916-40), Ed Muskie (1958-80) and her model, Margaret Chase Smith (1948-72), whose seat she now holds.

Collins’ popularity is partly a function of her caution. She receives votes from Republicans, Democrats and independents because she’s not given to fiery statements, lines in the sand and anti-government agitation.

Yet with Republicans controlling Congress, and Donald Trump in the White House, Collins occupies a pivotal position concerning health care, and the drive to “repeal and replace” the Affordable Care Act that’s dominated the new Congress.

Collins didn’t vote for the ACA, or “Obamacare” — unlike Snowe, who supported it as a member of the Senate Finance Committee, saying, “I do happen to believe that we have to move the process forward, on this very difficult and challenging issue.” Yet Snowe ultimately voted against the ACA on the floor, after failing to find a single Republican — even Collins — to join her.

Last week, Collins, in a crucial test, also voted to “move the process forward” — by supporting the budget resolution that will allow Republicans to repeal much of the ACA by majority vote, rather than regular legislation requiring support from Democrats. Will she continue voting for repeal?

It’s a tough question to answer, because Collins, like many Republicans, has been forthright about objecting to the ACA, but not about what, exactly, is wrong with it. She knows that the most “unpopular” feature, the “mandate” that everyone have insurance or pay a fine, is essential to keeping any private insurance system going.

When she’s criticized the ACA, it’s been on points such as the “cliff” that awaits those who make slightly more money than allowed for subsidies, and hence aren’t eligible for federal assistance.

Yet Collins is also aware that fixing the cliff would involve reducing subsidies overall, or spending more money to finance a gradual phaseout — hardly something high on the Republican agenda.

The cold, hard fact is that if Collins wants to protect health insurance for the 83,000 Mainers now covered through the federal exchange, something very much like the ACA will have to remain. The only real alternative is a national, universal system that’s not even in her party’s lexicon.

Congress will next consider the varied statements of Donald Trump on health care, including his vow to have the government negotiate drug prices with manufacturers. That would overturn the prohibition on lower prices Republicans installed earlier.

Trump also says he’ll provide “insurance for everyone,” but not national health insurance. Unlike Congress, Trump still resides in the world of magical thinking, where he can solve problems simply by waving a wand.

In short, if Collins wants to protect Mainers, and Americans everywhere, who will lose their health insurance through repeal, she will have to oppose the dominant strain of her own party.

Margaret Chase Smith did just that when in 1950 she denounced Joe McCarthy, a fellow Republican, on the Senate floor, for his accusations that the Truman administration was riddled with Communists — a charge McCarthy knew was false, but benefited him politically. After all these years, it remains Smith’s defining moment, the reason why she’s remembered when so many other senators, eminent in their time, have faded into the past.

Susan Collins now confronts a similar moment: Will she find the courage to speak truth to her party, and the nation, when it’s most needed? We’ll soon know the answer.

Douglas Rooks has covered the State House for 32 years. His first book, “Statesman: George Mitchell and the Art of the Possible,” is now available. Comment is welcomed at: drooks@tds.net