U.S. Sen. Susan Collins, R-Maine, speaks in Bangor on Nov. 4, the day after her re-election victory. Robert F. Bukaty/Associated Press

U.S. Sen. Susan Collins was sworn in to a historic fifth term in office on Sunday after having battled through a fiercely contested race to win re-election in November.

Former U.S. Sen. and Secretary of Defense William Cohen escorted Collins to her swearing-in ceremony in the Senate chamber, along with her husband, Tom Daffron.

“I want to express my heartfelt gratitude to the people of Maine for once again giving me the opportunity to serve,” Collins said in a statement. “It was an honor to be accompanied to the swearing-in ceremony today by my friend, Secretary Cohen, a dedicated public servant who provided our state and our nation with many years of dedicated service. I look forward to continuing to represent Maine in the U.S. Senate, and I will work as hard as I can each and every day for better health care, more jobs, and stronger communities.”

Cohen, a former Republican U.S. senator from Maine, mentored Collins when she served as a staffer on Capitol Hill. She won a race for Cohen’s seat in 1996 after he retired. At the time, she pledged to serve at most two terms in office.

Collins’ victory in November over former Maine House Speaker Sara Gideon, a Freeport Democrat, makes her the first popularly elected senator from Maine to win a fifth term. She is also the first Republican female senator elected to a fifth term in U.S. history.

Viewed beforehand as critical to Democrats’ hoped-for victory in the Senate, last year’s campaign saw heavy outside spending and impassioned opposition to Collins’ votes to confirm Supreme Court Justice Brett Kavanaugh and approve the 2017 Republican-led tax bill.

In a statement Sunday, Collins’ office noted that she won 14 of 16 Maine counties in the election last fall, and that she has now won re-election three times when her state voted for another party’s presidential candidate. The overall margin in the Nov. 3 popular vote, 51-42 percent over Gideon, was tighter than in 2014, when Collins won with 68.5 percent to 31.5 percent for Shenna Bellows, then a former executive director of the American Civil Liberties Union of Maine and last month elected as Maine secretary of state.

Collins has never missed a roll-call vote, reaching a total of 7,554, her staff said.

Once the latest in a long line of Republican moderates from New England, Collins is now her party’s only member of Congress from the region.