Rep. Sara Gideon, of Freeport, who has been the speaker for the past two terms, has already made access to abortion a central focus of her campaign. Collins, first elected in 1996, is expected to run for another term but has yet to formally announce her candidacy.
Collins cruised to re-election in 2014 and has long been viewed as a moderate Republican. Liberals set up a fund to help try to unseat her after she voted to confirm President Donald Trump's Supreme Court selection Brett Kavanaugh last year.
The fund has raised more than $4 million amid fear from liberals that Kavanaugh will vote to severely weaken access to abortion.
Gideon's website pledges to "put Maine First" and says she has "always stood up for a right to choose." The newly Democratic-led Legislature in Maine this year has passed laws to allow state Medicaid funding for abortions, and to allow advanced clinicians such as nurse practitioners to perform abortions.
Gideon, is also touting legislation to protect affordable health care coverage amid efforts to repeal former President Barack Obama's signature health law. Maine lawmakers this year passed bills aiming to lower prescription drug costs and ensure that Obamacare protections for individuals with pre-existing conditions are enshrined in state law.
"At one point, maybe Sen. Collins was different, but she doesn't seem that way anymore: taking over a million dollars from drug companies and the insurance industry and voting to put Brett Kavanaugh on the Supreme Court," Gideon said in a video.
Republicans in Maine and elsewhere immediately began trying to paint Gideon as a "far left" Democrat.
Maine GOP Chair Demi Kouzounas said Gideon is "eyeing Maine families' bank accounts to fund her extreme policy proposals."
Republican National Committee spokesperson Nina McLaughlin called Gideon a "rubber stamp" for Senate Minority Leader Chuck Schumer, who represents New York.
"From leading a corrupt caucus, to her inability to work across party lines even though she touted her ability to do so, Sara Gideon is just another far left Democrat who won't rest until she can give Mainers' voices and votes to New York," McLaughlin said.
Republicans are also pointing to Gideon's support of bills to provide assistance to noncitizens and switch from the Electoral College to a national popular vote. The voting bill died, while lawmakers pushed back a decision on the welfare bill.
Progressive activist Betsy Sweet has also announced a run against Collins.
Collins reported in April that she has $3.8 million on hand as she gears up for the 2020 election.
Collins' campaign noted that Democrats still have a year to go ahead of the June primary. Maine lawmakers this year passed a law to change from a presidential caucus to a statewide primary that supporters say will get more voters involved.
"One of the reasons why Senator Collins has been so effective is that she has more seniority than any U.S. senator from Maine over the past 70 years," said Kevin Kelley, spokesman for Collins for Senator. "She will continue to build on her record of extraordinary accomplishments for the people of Maine."
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