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(CNN) —  

Ever since Susan Collins cast the deciding vote to confirm Brett Kavanaugh to the Supreme Court last October, Democrats have pledged that they will beat the long-serving Maine Republican senator in 2020.

On Monday, they took a major step in making good on that promise as Sara Gideon, the speaker of Maine’s state House, announced her candidacy. “Susan Collins has been in the Senate for 22 years,” Gideon says in a video announcing her candidacy. “And at one point, maybe she was different than some of the other folks in Washington, but she doesn’t seem that way anymore.”

In the video, Gideon hits Collins for voting for President Donald Trump’s tax cuts and for Kavanaugh, and even features footage of the President thanking Collins. So you know where Gideon is headed in this campaign: Collins likes to say she’s a moderate but when she’s in Washington she votes like a Trump conservative.

That argument – minus the “Trump” part – has been tried before against Collins. In 2002 and 2008, Democrats ran serious, well-funded candidates against her. And attacked Collins as less moderate – and less in line with Maine’s values – than she tried to paint herself. Collins won with 58% of the vote in 2002 and 61% in 2008. In 2014, Democrats barely challenged her, and she was reelected with 67%. 

Collins is an able campaigner – and she’ll have as much money as she needs from Washington Republicans. (The Gideon video features Senate Majority Leader Mitch McConnell making that point.)

That said, there’s reason to believe that the 2020 race – now that Gideon is in – will be Collins’ toughest since she won the open seat of retiring Sen. William Cohen in 1996.  Here’s why:

1) Trump’s presence in the White House has radicalized voters into partisan camps, leaving very few of the centrists who Collins has always relied on. And if people vote purely on partisanship, Collins loses in Maine; Hillary Clinton carried the state 48% to 45% over Trump in 2016.

2) Collins’ Kavanaugh vote is just the sort of thing Democrats have been waiting years for. In past campaigns, Democratic strategists always struggled to prove – beyond any reasonable doubt – that Collins was more aligned with national Republicans than she let on. But they could never find that singular vote – until Collins voted to confirm Kavanaugh.

3) National Democrats – and the party base – are BEYOND fired up about beating Collins. In the wake of the Kavanaugh vote, more than $4 million was donated to benefit the eventual Democratic nominee against Collins.  Of the 22 Republican Senate seats up in 2020, the party’s base cares more about Maine than any other.

The Point: Collins is a survivor – as proved by her long electoral record of vanquishing highly touted Democratic candidates. But there’s reason to believe 2020 poses a challenge Collins has never seen before – and that might be too much even for her.