It can be found in that context. But it is surely not the most recognisable symbol for Antifa in the US.
The usual historical explanation first: 'Antifa' means primarily 'anti-fascist', a thing the US was proud to be, officially – not that long ago.
(src1 D-Day, src2 Operaion Bagration second source needed for accuracy. Click on smallest pictures throughout answer for larger versions.)
As the campaigner tried to explain on twitter with a more elaborate version of downward 'red triangle', it is found occasionally in an antifa-context:
Design details: Inverted red triangle, anti-fascist symbol, with the word ANTIFA
Tags: social rights, anti racism, anti nazi, human rights, inverted red triangle, freedom, antifa, rights, justice, anti nazis, anti-fascist triangle, equality, anti racist, No Trump, antifascist, against right, against fascism, socialism, red triangle, anti-fascist movement, peace, communism, demonstration, social justice, movement
– Spreadshirt: Posters "Antifa" by Jevaz archived.
Note that this vendor/designer claims to be from Spain and seems to also promote anti-fascism and feminism on his shirts. This would make uploading this just in response to allegations against the campaign quite an elaborate cover-up. But it's also claimed that the designer joined only two weeks ago, without further proof on dating this occurrence. This is said one may be online solely in response to these news. This uncertainty in the timeline would be the weakest point of this answer — and it is almost the weakest point of the response from the campaign as sole evidence for prior use.
The designer of this particular image is himself on Twitter and had this to say for dating the particularly 'quoted by campaign' design:
Hi. It's a design I recently created to market on a print sales platform. The inverted red triangle that is creating so much controversy is an anti-fascist symbol that has its origin in the way concentration camp prisoners are marked.
2:36 am · 19 Jun 2020 [my emphasis, LLC]
Since Twitter is an unreliable platform, from his account:
Yesterday a Trump affiliated account used my #Antifa design with a red triangle inverted and I was accused of being in the service of Trump's campaign, nothing more far be it from my intention that to seem affiliated with Trump or to offend the #Antifa movement.
Today is a symbol used by many #antifascist groups and politicians although it seems that in the USA it is not yet very widespread.
But apparently the Spanish designer Jevaz used the red triangle design element in this context at least as early as May 21, promoting it on Twitter. (Design 'Feminista' with text: "I think therefore I'm anti-fascist"). And in fact the German version of this Spreadshirt store has customer reviews for this design and designer from as early as
12. Januar 2018
Sehr schönes Shirt mit tollem Motiv, fällt allerdings etwas zu eng aus, vor allem an den Ärmeln. Daher empfehle ich eine Größe mehr zu bestellen.
Although it is not the only use of that symbol anyway, most often 'documented prior art':
A shirt design matching the ad campaign would then be
— CafePress: Red Triangle Gifts
But the actual product page seems to experience a few difficulties right now.
However, there are more of those, elsewhere, and definitely prior to the campaign:
– Antifascist Red Triangles (antifa left symbol) Sticker
Designed by reydefine
To claim like the campaign that this would be the symbol, or even a really common one – in the US – seems to not have been true. This changed obviously in a reverse-Streisand now.
In Europe, this is different. The red triangle as such was used in May First strikes in Paris in 1890 and has become a symbol of resistance to far right ideas, especially in Belgium. Like so:
– Triangle rouge, Les Territoires de la Mémoire
It is also the logo of the Ras l'front network in France. (WP: Triangle rouge).
The description for that network then reads as follows:
The purpose of this organisation was to create and stimulate a "movement of resistance and vigilance" against "fascism". It was aimed at citizens, associations, trade unions and political parties in order to combat the National Front, its allies and parties considered to be defending the same ideas. This network wants to be independent, both politically and financially [ref. necessary]. It is composed of autonomous collectives, and thus acts locally. (WP)
Such symbol usage as used for an antifa conference in 2018 Amsterdam:
For the local antifa group in Utrecht, notice that all others seem to prefer different symbols, and the now often mentioned Vrije Bond (Free Union):
Or another conference, in 2015 Munich:
– Antifant: Podiumsdiskussion zu Antifa und Organisierung 29. Oktober 2015
And perhaps the best known usage of this symbolism, if not the prior origin of using this symbol, again, before the Nazis used this for prisoners:
– Lazar Markovich Lissitzky (El Lissitzky): "Клином красным бей белых!” ("Klinom krasnym bey belykh! — Beat the Whites with the Red Wedge"
This was then used as inspiration in 2017 after Charlottesville:
– Shama Rangwala: "Links in the Wake of Charlottesville"
In the UK Anti-Fascist Action seems to have used this, uploaded in 2016:
Badges that reads ” Anti Fascist Action”
As part of our office clear-out, volunteers found a huge cache of campaigning postcards and badges mainly from the 1980s and some even earlier.
Sometimes it's said on Twitter this would be an isolated occurrence that should not be used as evidence in this debate. Isolated, it is perhaps not so much, as the following image was uploaded under the name of 'Antifa-Keil' (Antifa wedge):
– "Leo Trotzki über Faschismus", Der Funke, 25.01.2017
This wedge is even part of that paper's logo.
A book from 1980 uses the following cover:
— Wolfgang Abendroth, Walter Kreck, Reinhard Opitz & Max Oppenheimer: "Wie Faschismus entsteht - und verhindert wird. Materialien vom Antifaschistischen Kongreß Mannheim", Röderberg, 1980.
Also in London we could see this version of red triangle as used in 2015:
– Anti-Fascist Network: "Update on anti-fascist mobilisations this Saturday", 1.7.2015.
That still doesn't make any triangle the most widely used and most recognisable symbol of/for 'Antifa'. That would look like a bit more in line of those found in this picture:
– Gregg Re: "Portland, Oregon, pulls out of FBI terrorism task force, following San Francisco's lead", Fox News, February 13, 2019.
Antifa isn't a closely organised single group. There is no central structure. As such the single groups may choose any branding for themselves they like best. 'Two flags', '3 arrows', perhaps a 'fist, smashing things', 'hammer and sickle', even a 'swastika, but struck out'. That makes describing any of these antifa groups via one symbol quite complicated.
— Eva Steibl: "Antifaschistischer Spaziergang", uploaded November 14, 2018.
The East-German Kommittee der Antifaschistischen Widerstandskämpfer used this:
— Pin Triángulo Rojo - Condición Nuevo
El triángulo rojo invertido fue el símbolo que el régimen nazi usó para marca a los presos comunistas. Su uso es un homenaje al antifascismo. Pin metálico de gran calidad, pensado para coleccionistas. Medidas: 1cm
The only antifa group that I know that clearly does use the 'inverted red triangle' as their symbol is the VVN Vereinigung der Verfolgten des Naziregimes – Bund der Antifaschistinnen und Antifaschisten:
We see a red triangle used by the communist antifascists before there was a nazi concentration camp. We see that triangle used by nazis on their political prisoners. We also see some smaller groups using these concentration camp triangles for their own purposes after the nazis, like exactly the same with 'gay rights' groups sometimes using the pink triangle. The biggest and most organised antifa group in Germany does use the red triangle, officially in strict perpendicular orientation and in remembrance of the nazi-use.
That means that technically, the Trump campaigners have a grain of truth to work with. That it evidently failed so spectacularly is precisely because in the US the red triangle is not as common as it is in Europe and when most people do not recognise the symbol.
Notabene: 'Antifa in the US' in the sense of 'Antifaschistische Aktion' from Germany or the later and much broader antifa movement are a lot of only broadly similar things, lumped together. It seems obvious that in the US 'antifa' as self-designation in that precise sequence of letters only applies to a more militant faction of 'the left'.
This is decidedly not the case in Europe. While probably a lot of those groups can also be counted towards 'militancy', many, like the French or Belgian examples, and the German VVN example are not. The VVN being a registered club proper with actual membership and for a long time a tax-exempt charity.
While the above example images are evidence for 'claim is true' for the very wide international field of 'antifas', in the wider emaing of antifascists, including the militant ones, I simply failed to locate a single example image that's clearly for 'claim is true for the antifa (US)' or 'antifascists in the US', just variations like that one against the Ku-Klux-Klan hood, which was not an emblem for any 'antifa' directly.
Murtaugh is apparently on record with: “But it is ironic that it took a Trump ad to force the media to implicitly concede that Antifa is a hate group,”
Snopes doesn't get it right there: It's not that facebook said 'antifa be a hate-group' (whatever regurgitating news made of it), offical facebook said that 'the nazis' are the hate-group that used that symbol (for prisoners)… Whether facebook is correct or not, Murtaugh twists these words as if he really doesn't understand?
Their 'rules' (which they enforce quite arbitrarily): It's said that 'you may use a hate-symbol' in a context to say 'this is bad' and 'analyse/discuss it'. Just not without any of such context or 'this is great'… “Our policy prohibits using a banned hate group’s symbol to identify political prisoners without the context that condemns or discusses the symbol.”
Notice that it's 'the group' that's banned, then 'the symbol they used', not 'the symbol as such'. In effect, Facebook says: the group 'Nazi-Germany' is the hate-group — and that Nazi-Germany used that symbol to identify prisoners', and that that kind of usage requires 'condemnig, according to their rules.' This is now going full circle to the start of this answer.
The real irony is that eg the VVN (victims of nazism, not 'a hate-goup') usage would fall foul of these 'rules' as formulated, most of the time, as they use their symbol without always 'discussing it'…