The pin message in question almost certainly came from a place with a large Slovene population.

The biggest center at the time was (and still is) Cleveland, Ohio (estimates are that Cleveland has over 80,000 Slovenes, the largest population outside Slovenia itself). This wasn't necessarily so from the beginning of immigration -- when Slovenians immigrated (the biggest influx was from 1880 to 1914 with labor looking for work), they went to places with mining and factories like Omaha, Nebraska and Pueblo, Colorado -- but Cleveland was also included, especially for the Newburgh Steel Mill.

The steel mill became a center of gravity. The immigrant John Bradac came in 1890 and stayed in the neighborhood of the mill for 57 years, opening two saloons and helping 800 other Slovenians in getting citizenship. Other factories that opened were American Steel and Wire and Cleveland Rolling Mills. The steel plant had such a population of Slovenes that they had their own company band playing the music of the homeland. In 1919 Newburgh became the location of the Newburgh Slovenian National Home; it was expanded in 1949 with bowling alleys (still in use) and a large ballroom.

This isn't a guarantee that the button was from there -- Minnesota had (and has) a large population and additionally a popular Democrat, in the US House of Representatives, John Blatnik, who was in office from 1947 all the way to 1974. Blatnik was born to Slovene immigrant parents and during WWII worked with the OSS's division connected to the partisans of Yugoslavia. After leaving office Blatnik was replaced by Jim Oberstar, a staff member with Blatnik for many years and another Democrat-Slovene, who lasted in the office until 2011.

Generally speaking, due to "working class roots", Slovenians leaned Democrat in the time of Nixon, only starting to lean Republican with Ronald Reagan (not necessarily switching parties, but there were a fair number of "Democrats for Reagan" in 1980 -- Carter was just a turn-off for many and the economic woes in the country in general contributed to this). Not every prominent Slovene politician was a Democrat, the most notable exception being George Voinovich, who attended Nixon's inaguration and served in the Ohio House of Representatives (67-71) as Mayor of Cleveland (1980-1989) as Governor of Ohio (1991-1998) and eventually US Senator (1999-2011). He was one of the top choices to pair with Bob Dole on the failed 1996 presidential ticket, and he is why we have a picture of Bob Dole after dancing the polka.

For the meaning of the button it's simply: we're Slovenes, we're Democrats, and aren't you glad Nixon isn't Slovene? ... because that would be embarrassing and wrong.

...

Be sure to look at the other answer from /u/indyobserver!

Benedetto, R. (2006). Politicians are People, Too. United States: University Press of America.

Dutka, A. F. (2017). Slovenians in Cleveland: A History. United States: Arcadia Publishing Incorporated.

Pauwels, C. L. (2009). Historic Warren County: An Illustrated History. United States: Historical Pub. Network.

While I like /u/jbdyer's answer - I hadn't even thought about that angle - there's an even more obscure possibility: that it was referring to the moral fiber displayed by Vladimir Pregelj, the Slovenian-born foreman of the grand jury tasked with indicting the Watergate conspirators, including Nixon.

So a major caveat: campaign paraphernalia can be a very oddball thing to research as they are often a cheap source of fundraising based on partisan flash-in-the-pan issues that sometimes never get much media traction in the first place, let alone academic attention afterwards. I've touched previously on one aspect of the 1916 Wilson-Hughes race being a proxy for American entry into World War I: the long forgotten McLemore amendment, which would have prohibited Americans from traveling on armed American merchant ships. Surprised you've never heard of it? It's probably because it was irrelevant before and after the election, but during a brief couple of months the fights over it in Congress on it were as bitter and divisive as anything before or since. I wouldn't be at all shocked if there are some decaying campaign buttons or ribbons talking about "Courage for McLemore!" or such, but unless you are very familiar with the Wilson era - it takes a deep dive in the academic literature to catch it - it's a stupefying reference.

Nearly the same goes for Vladimir Pregelj. He shows up nowhere in the Nixon or Ford literature, nor in most of the plentiful Watergate material. The first comment I could find on him was from Emery's 1994 Watergate, where he appears in a brief reference as the foreman who presented the sealed indictment of Nixon in March 1974 as a secret co-conspirator for obstructing justice to Judge John Sirica, publicly indicted the main players like Haldeman and Erlichman, and recommended turning over the evidence they'd reviewed to the House Judiciary Committee.

Grand juries are supposed to be secret, but prior to this later appearance (where the grand jurors were "dressed to the nines; many of the women wore their favored church hats"), back in July 1973 Sirica had assembled the panel publicly and Pregelj suddenly became the most prominent of them. From a very recent book, Watergate: A New History by Garrett Graff:

"In a regular ritual of the American justice system, made unique only by their target, the grand jury, one by one, asked the court to order the compliance of the president. It was the first time the country had glimpsed the men and women sitting in judgment on the case. Sirica’s clerk struggled in the moment to pronounce each name, and so the foreman, Vladimir Pregelj...offered to help. Pregelj, the papers noted, had a large Van Dyke beard and impressed observers with his “muted but mod clothing.”"

While at least he didn't have social media accounts to lock down, the striking figure of Pregelj started receiving hate mail almost as quickly. That grand jury went through copious amounts of material (they met over 100 times between July and February) at immense personal cost to many members of the panel; they were paid $20 a day unless they were in part of the ten or so like Pregelj who were federal government employees who got paid their normal salary. Pregelj was one of the few with an advanced education - a refugee, he worked for the Library of Congress as an economist and Slovenian specialist and produced several CRS reports in the following years - which is probably why he was named foreman by Sirica. One of the most notable actions he took was after a 19-0 vote by the grand jury to personally write Nixon on their behalf asking him to testify before them; Nixon declined.

Now, one issue with my speculation is that just how widely Pregelj's actions were known at the time is a bit beyond the scope of this answer. In theory, it's modern enough so that if you were really interested you could start trying to compile media mentions and impact scores and such, but the main problem is that I'm not at all sure even if someone proved that he fully had his 15 minutes of fame that it'd be definitively revealing about a link between him and the buttons. However, given they appear to have been printed right around Watergate, I do think there's a pretty decent shot that they could refer to him in some capacity. It might very well be that Slovenian-Americans printed those buttons up to provide contrast of the slimy Nixon to an upright Slovenian hero, it could have been others that were impressed with the tenacious grinding work of the grand jury and held him up in contrast, or it's possible I'm completely wrong on an interesting but unrelated tangent.

In any case, at the very least it's worth noting, since the man deserves to be pulled out of obscurity.