ORONO, Maine — When the University of Maine on Thursday unveiled the world's largest boat created by a 3D printer, the school turned to the state's senior U.S. senator to smash a bottle of champagne over its bow.

It's tradition to christen a new boat by busting a bottle of bubbly, and this was no ordinary vessel: The boat was also produced by the world's largest 3D printer, introduced the same day.

Who better to bust the bottle than Sen. Susan Collins, the four-term Republican senator who helped secure the project's $20 million in federal funds through her senior status on the Senate Appropriations Committee?

"It's such an exciting day," Collins said.

But beginning Tuesday, it's goodbye to the feel-good ceremonies back home in Maine as Collins, 66, returns to Washington for what could be a raucous stretch. Greeting all members of Congress will be an issue that's engulfed the Beltway, one that looms particularly large over Collins' reelection next year — the House's impeachment inquiry into President Donald Trump's interactions with Ukraine.

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Collins, who is seeking a fifth Senate term next year, was already facing the toughest race of her Senate career. She's among the top targets for Democrats nationally in their quest to reclaim control of the Senate. The race is expected to be Maine's most expensive ever, with nonpartisan ad tracking firm Advertising Analytics projecting $55 million in overall spending.

Long considered part of a disappearing breed of moderate senators with a reputation for bipartisanship, Collins has felt increasing heat from the left and even some moderates. Last year, Collins drew the scorn of Democrats nationally for her vote in favor of the confirmation of Supreme Court Justice Brett Kavanaugh. The year before, it was Collins' vote for Trump's tax cuts that angered Democrats.

But perhaps no other issue will test whether Collins can maintain a middle lane more than a Trump impeachment, which would eventually force her to pick a side if it goes to the Senate for trial.

Collins 'amazed' colleagues have mind made up on impeachment

For now, Collins is walking a fine line, not saying whether she supports the merits of the House's impeachment inquiry but also condemning the president's public assault on the federal whistleblower as "wrong." Whistleblowers should "be supported and encouraged and not denigrated," she said.

Whether to bring articles of impeachments "rests with the House," Collins told reporters following the 3D printer and boat event. She's cast herself as a juror who is waiting to review the facts if and when impeachment goes to trial in the Senate.

“I am amazed that some of my colleagues have already made up their mind one way or the other before all of the evidence is in and before the facts are known," Collins said. "I think that’s entirely inappropriate, whether they are for the impeachment or against the impeachment.

"Under the constitution, the role of the senator is to act as a juror, and that is what I did in the case of the impeachment trial of Bill Clinton. And should this come before us, which I think it will, that’s what I will do here.”

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Collins declined to take further questions from USA TODAY about impeachment. A Senate aide cited time constraints.

In response to a question posed by email, Collins said her constituents are more concerned about the high cost of prescription drugs and dysfunction in Washington.

"I have heard comments on both sides of impeachment," she said, "but the majority of comments have been on issues that directly affect Mainers’ day-to-day lives.”

Earlier this month, Collins said she was "stunned" when Trump invited China to investigate former Vice President Joe Biden and his son Hunter Biden. She called it "clearly wrong." The impeachment inquiry started after a whistleblower letter accused Trump of pressuring Ukraine to investigate the Bidens.

Her office did not answer emailed questions seeking Collins' position on the White House's refusal to comply with the impeachment inquiry and whether she found the transcript of Trump's phone call with Volodymyr Zelensky appropriate.

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Collins' Senate colleague from Maine, Sen. Angus King, an independent who caucuses with Democrats, has taken a much different approach: embracing the inquiry. In an op-ed published this week in Maine's largest newspapers, King wrote "it’s hard to argue that beginning a formal process to get all the facts is unjustified."

Support from Collins would likely be key for the Senate, controlled by Republicans, to remove Trump from office — an action that right now looks like a long-shot.

In 1999, Collins, then a first-term senator, voted not guilty on both articles of impeachment against Clinton after the Democratic president had been impeached in the Republican-controlled House. She remained mum on how she would vote until the end. But months before, she said the House has "a constitutional obligation to conduct an inquiry," the Portland Press Herald reported in October 1998.

Democrats line up to take on Collins

Collins, who still hasn't formally announced her 2020 run, won her last race in 2014 easily with 69% of the vote against the Democrat, former ACLU of Maine Executive Director Shenna Bellows. The victory was the most lopsided reelection in Collins' career. Her campaign already has $5.4 million in cash on hand for 2020.

Nevertheless, Democrats are energized to take on Collins, who they accuse of abandoning her independent tendency to shift to the right, underscored most by her Kavanaugh vote.

More:Democratic challenger criticizes Susan Collins after new Brett Kavanaugh allegations

Four Democrats have announced bids for the Democratic nomination, including Maine's House Speaker Sara Gideon, widely seen as the party's frontrunner. Other Democratic candidates are attorney Bre Kidman, progressive activist and former gubernatorial candidate Betsy Sweet, and retired Air Force general Jon Treacy. Conservative blogger Derek Levasseur announced a primary challenge against Collins but halted his campaign last month.

"Encouraging a foreign government to investigate political opponents is an egregious abuse of power," said Gideon, who supports the impeachment inquiry. "Congress should investigate and the White House should be forced to comply, not stonewall an effort to get the facts."

Two Green Independent Party candidates, David Gibson and Lisa Savage, and two independents, Tiffany Bond and Danielle VanHelsing, are running for Senate as well.

Independent and third-party candidates could factor into the election through Maine's ranked choice voting. Voters rank candidates on the ballot by preference. If no candidate gets 50 percent of the vote then the last-place candidate is eliminated and the second choices of voters who backed that candidate are counted. The process continues until someone reaches a majority to win.

Maine, a moderately blue state, but one with an independent streak historically, last voted for a Republican president in 1988. But the margin in 2016 was the closest since then with Hillary Clinton beating Trump statewide by only 3 percentage points.

More:Susan Collins is 'sad' her support for Brett Kavanaugh could cost her votes

For the first time since 1828, the state split its delegates. Clinton received three for winning statewide and Trump got one Maine delegate for winning the state's 2nd congressional district, which is made up by the rural, vast northern part of the state. The split underscored a growing political divide with the more progressive southern and coastal parts, including Portland, the state's largest city.

Impeachment decision bound to anger one part of the state

"Let's say she comes out opposed to impeachment when it's all said and done, she's going to anger one part of the state," Mark Brewer, a political science professor at the University of Maine, said of Collins. "If she comes out in favor of impeachment, not only is she going against her own party, which causes problems, she's going to anger another part of the state.

"So she's definitely in a tough spot and there's no doubt that impeachment makes it tougher."

And yet, Brewer stressed, just because the election will be tougher than past contests for Collins, it does not mean the margin will be close.

"I think you could still end up seeing (Sen. Collins) winning by a comfortable margin. On the other hand, I could also see it being much tighter. And you don't have to squint too hard to envision a case where she goes down in defeat."

Early polling has shown Collins comfortably ahead in a hypothetical match-up with Gideon by double digits. But a Morning Consult poll in July found reason for concern for the Collins camp and encouragement for Democrats. It showed 45% of Mainers approved of her performance and 48% disapproved, a net drop of 16 points since earlier in the year.

Collins did not vote for Trump in 2016, but she's avoided the attacks from the president that Senate Republicans who have criticized Trump have gotten. Despite drawing the ire of some hard-line Republicans over the years, Collins was endorsed in August by former tea-party aligned Republican Gov. Paul LePage. In the past, he's criticized Collins, including for announcing she would not vote for Trump.

Collins attracts both scrutiny and support from Mainers

A range of opinions about Collins are found a few blocks from the University of Maine campus, across the Stillwater River in downtown Orono, population 11,305, which voted for Clinton in 2016.

Taylor Hamilton, a 28-year-old college student and self-identified independent, who said he voted for Collins in 2014, said he will likely vote Democrat next year. Still, he applauded the senator for being "unbiased" and open-minded on investigating Trump.

"I agree with some of her policies, but most recently, she's favored siding with the Republicans more so than maybe her constituents," he said, pointing to her 2017 vote for Trump's tax cuts. "I think I would prefer a Democrat over her."

Inside Pat's Pizza, still a popular restaurant 88 years after it first opened, the talk is different.

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June Smart, 75, said she voted for Collins in the past but wants a stronger Republican who will get more forcefully behind Trump. Now retired, Smart previously owned a trucking and wood-chipping company in the area with her now-deceased husband.

"I liked her at first but I think she's waffling to the other side, to the left," Smart said. "I don't know if I would vote for her again."

But a few booths away is Doug Marchio, 74, a retired former IT specialist at the university. He lauded Collins as a "tempering force" in Congress and said most Mainers view her the same.

"I think people have a lot of history with her, confidence in her and feel they know her character," he said. "My own guess is she will be A-OK in the election."

Contributing: Associated Press

Reach Joey Garrison on Twitter @joeygarrison.