As part of the deal for democracy get wide amnesty of past crimes and reorganize as a populist political party. Then make the continued rejection of this minority as "permanent foreigners" your main political issue. From description it kind of is, anyway.
This kind of negative agenda is very easy to sell to people. Just make noise about any crimes committed by members of this minority and inflate any real fears and concerns people have about them out of all proportion. If you are this serious about keeping them as non-citizens you must have lots of material you can talk about with great passion and total sincerity.
Wrap it with nationalistic jargon about keeping the nation strong and unified. Talk about sacred cultural and religious values with great conviction.
Portray any attempt by other parties to give the group any rights as "selling out" the independence and sovereignty of the nation to foreign powers. This won't be that hard since foreign powers probably really will push the issue. And every time they do, your position will be stronger.
In general, push an anti-internationalist agenda. This is easy to sell to people and as it has been standard for pretty much every populist movement, you can easily copy from real world precedents. For example the British Brexiteers (and other euro-sceptics) or (if American) the way scepticism about the UN, various climate pacts, NAFTA and other large trade deals both give easy examples of how to use the natural suspicion people have of foreigners for political gain.
Another standard populist agenda is anti-elitism. You should adopt that as well. It is easy to do, allows you gain popular support while saying things that are just non-sense, and works with all issues. In your case "the elites" are also the second most likely group after "the foreign powers" to push for citizenship, so they are your enemies anyway.
For examples, you can use "the climate debate", again the Brexiteers, or the anti-immigrant propaganda anywhere. The way Trump handles the Mexican illegal immigrants is probably the easiest to copy. When you talk simple and loud and with great conviction about an issue that is really very complex, "the elites" trying to oppose you with facts will look weak and dodgy. The Brexiteers were very similar. Lots of things they said did not really make any sense but that does not really matter as long as it is simple and appeals to emotions.
As long as the new democratic parties are not united in a single bloc this will give you enough popular support to make giving citizenship to the minority extremely risky. If you have done your prep-work properly and made any democratic opposition to the regime properly fragmented, you have decent chance of making it impossible.
This is actually a fairly good approach to going democratic as the ethnic minority gives you popular agenda that is not directly connected to your non-democratic past. You basically get a strong new start in politics for free. It also plays well with standard populist politics that allows you to just copy already proven in practice to be efficient solutions from history books or from elsewhere.