Why were Maine presidential polls fine(in comparison) but had absolutely garbage senate polls?

[Bootes Void]

Biden got 53% in the presidential race which is what the polls expected but the Senate race was a complete flop. What happend?

[ProgressiveModerate]

Well, first off, there was a lack of Senate polls in Maine in the days leading up to the election, and the polls we got generally had high undecides leading to extra ambiguity. Maybe if the election had been held in September or October when Gideon was doing well in many polls, she might've won; it's really hard to tell. It's also important to note that while Collins won by a large margin, she only won 51% of the vote, and while we'll never truly know, it seems like RCV probably would've broken heavily for Gideon and it might've narrowed to only a 3 or 4% margin. If that were the case, those Gideon + 2 polls wouldn't have been any more off than the Kelly + 8 polls or the Cunningham + 5 polls.

[TML]

In terms of the presidential polls, it should be noted that this year they were off by about 10 points in northern Maine, just like they had been back in 2016.

[Tekken_Guy]

In terms of the presidential polls, it should be noted that this year they were off by about 10 points in northern Maine, just like they had been back in 2016.

And Jared Golden was getting those 20+ polls.

[Intell]

I'm sure ticket splitting Biden voters would be less likely to answer than straight-D voting Biden voters too. This likely explains the gap.

[Figueira]

I'm sure ticket splitting Biden voters would be less likely to answer than straight-D voting Biden voters too. This likely explains the gap.

The polls did show some late movement towards Collins, but not towards Trump, so it's possible that a lot of people changed their minds in the last few days and polls couldn't keep up.

[Amenhotep Bakari-Sellers]

The problem is that all of our female candidates like Bollier, Greenfield and Sara Gideon and a hostess of House female candidates lost badly. Pelosi was so confident about a Blue wave, she decided not to compromise on the stimulus and it would have helped out candidates to pass the 1.8T, the package she has cut to 900B without stimulus checks now

[Pollster]

The polls this year followed the same trajectory that polls in Collins' past races followed, with the difference this year being that the race was significantly more competitive than ever before.

Copying and pasting this post from a past thread on a similar topic:

2008

RCP average: Collins +14.6
Actual result: Collins +23

https://www.realclearpolitics.com/epolls/2008/senate/me/maine_senate-564.html

2014

RCP average: Collins +29.2
Actual result: Collins +37

https://www.realclearpolitics.com/epolls/2014/senate/me/maine_senate_collins_vs_bellows-4168.html

In both races [2008 and 2014], the Democrat performed almost exactly at their [polling] average and Collins wound up consolidating the undecided/uncommitted voters (probably Dem-leaners who didn't want to commit to voting Republican but were always more likely than not with Collins).

It looks like Gideon's polling suffered from both that Collins consolidation phenomenon, as well as pollsters generally modeling for a 2018-style environment, which likely made her average vote share a bit too high. If we assume that Gideon's average was a few points too high throughout the race, then the same "Dem hitting their average/Collins consolidating" pattern probably holds in this race as well. The fact that Collins' overperformance was roughly the same 8 points both times, and her final average (probably lowballed for the same reason Gideon's was overshot) this year was around 42% before she won with 51%, fits into this scenario almost perfectly.

A good (though obviously imperfect) historical comparison is NV-Sen 2010: incumbent is a multi-term institution with long history of consolidating in the end, polls modeling a different turnout than actually happened, giving the incumbent a lower share and the challenger a higher share than was the truth.