Washington (#CNN)Republican Sen. Susan Collins will announce Friday morning whether she will run for governor of Maine in 2018, a source with knowledge of the senator’s plans told CNN.Collins, among the most moderate GOP senators, has been a thorn in President Donald Trump’s side — particularly on health care, where she consistently voted against the GOP’s efforts to roll back former President Barack Obama’s Affordable Care Act.But were she elected, her departure from the Senate would be a sharp blow to Republicans because without Collins, the seat would become much more difficult to keep out of Democratic hands in 2020.Collins is spending the Columbus Day week-long recess making her decision.A week ago, she told Portland-based WCSH she was feeling the pull to the state she has represented in the Senate since she was first elected in 1996.“I really love being in Maine,” she said. “Going back and forth each week is difficult, and my family and friends are in Maine. I believe I could make a difference and job creation and economic opportunity in our state.”A source with knowledge of Collins’ thinking said over the last year Collins has taken a serious look at running for governor. Collins values constituent services, felt she could do more in an executive role and, the source said, liked the idea of capping her political career as Maine’s first female governor — the job she ran for unsuccessfully in 1994.But, the source said, the reality of Collins’ prominent role in the Senate — where she has seniority and is a swing vote on nearly all major legislative items — has led Collins to rethink whether she wants to depart now. Another factor in her thinking is that her friend Arizona Sen. John McCain is being treated for brain cancer — which, the source said, has made Collins’ voice countering Trump more important.Since New Hampshire Sen. Kelly Ayotte was defeated last year, Collins has been the lone Republican representing New England in the Senate.She has at times been a bridge between Republicans and Democrats, and has won praise for standing up to Trump — even as she also drew criticism from her Maine’s GOP governor, Paul LePage, for failing to support Trump’s legislative agenda.