Asked to imagine the US Senate without Sen. Susan Collins (R) of Maine, Sen. Chris Coons (D) of Delaware tilts his head back and places his hand on his chest – a gesture of both respect and sadness. “The Senate loses something big when it loses members who have been here a long time – who have great sense, who are well liked and widely respected, and who want to get things done,” he says.

He was speaking not only about Senator Collins, who is expected to announce this week whether she will run for governor of her home state, but also about retiring Sen. Bob Corker (R) of Tennessee, the plain-speaking chairman of the Foreign Relations Committee, who became embroiled in a tweet feud with President Trump over the weekend, and of Sen. John McCain (R) of Arizona – an institution unto himself, who is still working while having treatment for cancer but who has called his prognosis “very poor.” Last week, Senator McCain told reporters he was feeling good and then joked, “really good until I ran into you all.”

Collins, Corker, and McCain are three Republican pragmatists who regularly work across the aisle. Their exits – along with the potential loss of several endangered Democrats up for reelection in red states – would all but complete the hollowing out of the Senate’s political center. It’s a trend that’s been going on for more than a decade on both sides, says Jennifer Duffy of the independent Cook Political Report: “There’s virtually no middle left in the Senate.”

The power of the center

Ironically, the center is a powerful place to be for Republicans these days. With a slim majority of 52, it takes only a few wayward votes to make or break the president’s agenda. Collins was one of three Republicans to sink GOP efforts to “repeal and replace” the Affordable Care Act. Spontaneous applause broke out in the Bangor airport when she returned home after the vote. McCain, who insisted on a more considered, collaborative approach to health care, was another consequential “no” vote.

A tax overhaul, the next big GOP agenda item, is now in danger. Senator Corker has made it clear he will not support the Republican tax plan if it adds “one penny” to the deficit.

Corker has also become a prominent voice of concern over the president’s tweets and rhetoric. He questioned Mr. Trump’s “stability” and “competence” in the wake of Trump’s comments about the violence in Charlottesville, Va., in August, and last week he told reporters that only a handful key officials are separating the country from “chaos.”

This sparked a Twitter lashing from the president over the weekend, saying Corker “didn’t have the guts” to run for reelection, with Corker shooting back that the White House is akin to an “adult day care center” where someone “missed their shift.” In a New York Times interview on Sunday, Corker said the president’s reckless threats could put the United States on “the path to World War III.”

Although a lame duck, Corker will be a crucial player in the upcoming debate about the president’s likely decertification of Iran from compliance with a nuclear deal, and in any Senate confirmations of ambassadors or a new secretary of State.

The power these senators hold by not always toeing the party line is not lost on them. As Collins weighs whether to run for governor, she is well aware of the influence she wields on key issues – from taxes and spending to North Korea and the Senate’s Russia investigation to health care.

“One of my colleagues who is trying to get me to stay in the Senate told me I am the pivotal player on a lot of important issues,” Collins told WCSH of Portland last week. Her mother agrees and thinks she should stay.

But 20 years is a long time to commute to an ever-more partisan Washington, where it’s become harder and harder to get anything done. And Collins has been asking herself how she can best help the people of Maine. A leaked internal memo shows her in a strong position to win a GOP primary, despite Republican Gov. Paul LePage’s sharp criticism of her stand on health care as “shameful.” Her job approval among Mainers is at a staggering 75 percent, making her the overwhelming favorite in a general election.

'Shoring up' the middle?

The thought of Collins returning to Maine sent Sen. Heidi Heitkamp of North Dakota – one of those endangered red-state Democrats – to her phone, where she texted her friend a simple plea: “Don’t do it.” The junior senator from Maine, Independent Angus King, who caucuses with Democrats and was once Maine’s governor himself, has also urged her to stay, pointing to her seniority as a plus for the state.

“The middle – it needs shoring up,” says Sen. Claire McCaskill (D) of Missouri, another red-state Democrat facing a tough reelection battle. Compromise is necessary, she says, and it’s what the Founding Fathers intended, “so when people leave here that get that part of the job ... it hurts the institution and it hurts the American people.”

Not everyone agrees that the departure of Collins, Corker, and McCain would snuff out the last flames of bipartisanship in the Senate. On a pleasant, sunny day last week, Sen. Bill Cassidy (R) of Louisiana sat on a bench on the Capitol grounds and went through some mitigating factors.

To begin with, Corker and McCain head committees that are traditionally bipartisan in nature (McCain chairs the Armed Services Committee). Those committees can be expected to continue to work on a bipartisan basis, says Senator Cassidy, who was the co-author of “Graham-Cassidy,” the GOP health-care bill that failed last month. Earlier, he worked on a health-care bill with Collins that he thought might attract bipartisan support. It didn’t.

While these three lawmakers are known for seeking common ground – a “good thing,” says Senator Cassidy – he maintains that approach is not as rare as some might think: “For example, I’ve spoken to three Democratic senators today about collaborating on pieces of legislation.”

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It’s also impossible to predict what a new Senate’s makeup might look like or how individual lawmakers will behave once they’re in the office. Maine has a long tradition of sending independent-minded senators to Washington, but its internal politics are changing. Indeed, part of Collins’s calculation is whether Governor LePage or she would name her replacement. Collins favors a scenario in which she could name her successor, who would serve until voters decided in the 2020 election.

“Being a governor is kind of fun. You get a lot done,” says Duffy, weighing some of Collins’s likely considerations. “If she stays [in the Senate], she’s frustrated. She has a target on her back all the time.” The reward for that frustration, however, is power, at a time of great national consequence.