As republicans barrel ahead with plans to appoint a successor to justice ruth bader ginsburg on the supreme court, few politicians are being watched more closely than susan collins, the us senator for maine.

Ms collins, the last republican member of congress from new england and one of the partys few remaining moderates, was already facing the toughest re-election battle of her career, in part because of her support for donald trumps last appointment to the court, brett kavanaugh.

On saturday ms collins issued a carefully worded statement in which she said she would have no objection to the senate judiciary committee reviewing the credentials of the us presidents nominee before the election. but she added: in fairness to the american people...the decision on a lifetime appointment to the supreme court should be made by the president who is elected on november 3.

The hedged language reflects the difficulties that ms collins faces as one of the last moderate republican lawmakers in the age of trump.

On monday sabatos crystal ball, an influential election forecast, moved ms collinss race from a toss-up to lean democrat. a quinnipiac university poll released last week before ginsburgs death showed ms collins trailing her democratic challenger, sara gideon, by 12 points.

Other polls suggest a tighter race, but with ms collins still behind. she trails ms gideon by six points in the real clear politics average.

Ms collinss fate has implications not just for voters in maine, but also for republican prospects of maintaining control of the senate, where they currently hold a 53-47 majority

A loss for ms collins could signal broader problems for the eight senate republicans who are also fighting to hold on to their seats, especially party moderates such as colorados cory gardner.

Ms collins is one of just two republican senators, alongside lisa murkowski of alaska, who has signalled she will not vote for a supreme court nominee before the end of the year. that position will probably play better for ms collins with democratic and independent voters, while potentially hurting her among the states republicans.

On monday mr trump attacked the maine senator in an interview on the fox news channel. i think susan collins is very badly hurt by her statement...people are not going to take this, he said.

Bar chart of polling (%) showing gideon extends her lead over senator collins

To the presidents most fervent supporters, ms collins, who was first elected in 1996, is a relic who failed to support him in key moments, such as when he attempted to dismantle the affordable care act.

To democrats, ms collins, 67, has been an even bigger source of disappointment: one of the few republicans willing to express concern about the presidents behaviour who nevertheless voted against his impeachment and rubber-stamped his nomination of mr kavanaugh to the supreme court.

In this campaign, ms collins has attempted to play both sides, painting herself as a problem solver who seeks common sense solutions. but that strategy may prove to be out of date.

The day when two to three moderates on the democrats side and two to three moderates on the republican side can work somethingout, and have an impact, is just gone, said tom allen, a former longtime democratic congressman from maine who ran against ms collins for the senate in 2008.

She would like to be...appreciated for being in the middle, but there isnt a middle any more that matters, mr allen added.

In the past, ms collins has been able to run ahead of her partys presidential nominee, winning her races even when bob dole and john mccain lost the state in 1996 and 2008, respectively.

But given how unpopular mr trump is with democratic voters, fewer may now be willing to split their ballots in her favour, according to amy fried, a political-science professor at the university of maine. she won the last timewith roughly 40 per cent of democratic support. thats clearlynot going to happen this time, ms fried said.

The senate race in maine is the most expensive in the states history. as of the end of june, ms collinss campaign had raised $14m to ms gideons $24m.

Supporters of ms collins defend the maine senator as the last of a dying breed: a true independent who has consistently ranked as one of the most bipartisan senators and never missed a vote.

They have touted ms collinss deep roots to the state, where she grew up digging potatoes for 30 cents a barrel. most mainers have already met senator collins, shes been to every town, said ben gilman, who has worked on republican campaigns in maine for decades.

Allies dismiss much of the criticism of ms collins as sexist, particularly the accusation that she is owned by republican senate majority leader mitch mcconnell.

While critics may decry ms collinss political calculations, allies say the republican senator had few other options for political survival.

Bob tyrer, who has known ms collins since the 1970s and managed her first senate race, said: if you spend your whole time trying to turn yourself into a pretzelto be popular, even the people who are quote-unquote happy with your vote...probably dont respect how you got there.