As long as main land China exists and is strong, DPP will get majority of votes.
It doesn't matter.
The foundation of all authority is the same in all cultures and all historical periods.
(1) security -> (2) stability -> (3) prosperity
Democracy only replaces armed challenge as means of contesting established authority. Elections are battles where instead of drawing blood we draw ballots. Opinion polls are tallies of armies that can be fielded by each faction.
As you well know armies can defect, desert or refuse to fight. If a war drags on a larger army might fail to perform and be beaten by nominally weaker adversary. I think the CPC might know a thing or two about that.
So here's how CPC could unseat DPP from power without winning an election on Taiwan.
Taiwan has several islands outside of the main island that are part of its territory. Kinmen, Matsu and Wuqiu are in very close proximity to the mainland. Taiping island is located in the middle of the triangle formed by the three large bases in the Spratly archipelago. Pratas island is closer to the mainland than to Taiwan. All of these islands are municipalities where KMT has majority support. DPP is strong only on Taiwan proper.
A provocation is staged on one of the islands - let's assume a pro-unification and pro-China protest - and KMT being a pro-unification and pro-China party does nothing beyond blowing some smoke in no particular direction.
Let's assume that the provocation is staged on Kinmen which is the largest and most populous of the islands as well as closest to the mainland.
The crisis puts the pressure on DPP to respond. After all a protest is a challenge to authority and if not addressed it will weaken (2) stability - thus affecting (1) security and (3) prosperity.
DPP must respond or risk losing political control of the islands. If DPP loses control then KMT might declare autonomy in those municipalities and stage a test run of peaceful reunification with the mainland as a special administrative region, even under a form of "one country, two systems" model. DPP must counter such possibility because a success of such unification will absolutely shatter one of the foundations of its political platform - that is the claim that Chinese control of Taiwan would result in repression of local population.
Because the protests are fundamental in nature and deliberately intended to aggressively test the government's response DPP responds by the use of force sooner or later. When DPP uses force rules of war are in effect and the protesters can stage an overt insurgency.
A hypothetical scenario:
1. A group of unidentified "little brown men" lands on the Taiping Island and declares it a sovereign territory of the Republic of Philippines. PLA forces in the area are put on high alert. The air force is patrolling the area to prevent any US special forces deployment. The "little brown men" seem like group of pirates or insurgents from one of the unstable provinces.
2. PRC coast guard ships intercepts several boats with the rebels attempting to capture one of the smaller artificial islands. A firefight ensues. A clip of the boats firing on coast guard vessel leaks to social media. PLAN sinks the boats and puts Taiping island under a blockade.
3. DPP sends whatever they can pare of the navy and marines to quell the unrest and secure the island. They can't send paratroopers because the "little brown men" almost shoot the plane down.
4. The rebels engage PLA forces before Taiwanese ships arrive at the island. PLA captures the island and establishes a temporary military peacekeeping operation. They do not declare the island as part of PRC territory.
5. Taiwanese navy arrives to Taiping island and demand entry. PLAN delays all responses until sufficient level of aggravation is achieved and the lets the Taiwanese navy through. Taiwanese military is allowed on the island and operates parallel with PLA forces as PLA units are nominally withdrawing from the island. In the media DPP declares Taiping island to be part of Taiwanese territory and demands that PLA forces leave its waters.
6. On Kinmen a demonstration turns into a riot as pro-China protesters demand DPP open negotiations with CPC regarding re-unification. They protest the declaration of Taiping as "Taiwanese" territory. Similar demonstrations are staged on Matsu and Wuqiu islands and on Taiwan proper - where plausible and applicable. A self-declared "Interim commission on the reunification of Kinmen" is established and they enter in negotiation with KMT authorities and KMT authorities only regarding opening channels with Beijing that circumvent Taipei. In short it is an open challenge and declaration of secession.
7. PLAN stages an impromptu naval exercise near the Pratas island isolating Taiwan and the naval forces in the Spratly islands. The PLA forces withdraw from Taiping island but not from the immediate area. Taiwanese navy is stretched between all possible locations undermining its logistical ability.
8. An incident on one of the islands where Taiwanese forces shoot a protestor is leaked to the media. China provides humanitarian aid and uses Hong-Kong tactics against DPP. More incidents happen and they are immediately leaked to the media. PLA does not get involved but it makes any logistics and intervention as cumbersome as possible. KMT is treated preferentialy by PLA in providing aid to islands under siege thus helping their image. As the DPP is trying to repress the island CPC provides KMT with platform to present them as reasonable and reliable partners. The primary goal of PLA presence is to prevent external actors from getting involved. China insists it is a matter that DPP, KMT and CPC must resolve on their own without "malicious interference from outside powers".
9. A brutal attack against pro-China protestors on Taiwan by a mob of pro-Taiwanese rioters is staged by agents provocateur providing a cue for KMT to start attacking DPP. They declare that DPP's grip on power and reality is slipping and that they are using foreign assets to attack political opposition.
10. As Taiwanese navy is forced to leave the Spratlys due to the situation in the north demanding more and more ships a group of "little blue men" repeats the takeover but this time they declare Taiping island to be a sovereign territory of China and claim allegiance to KMT. They seem to be former military and demand publicly that KMT enters official talks on re-unification with China of all the areas where Chinese people are oppressed under DPP regime. Not waiting for response from Taipei the "little blue men" place a KMT flag on Taiping island, declare it to be the territory of Republic of China and formally invite PLA commander for the Spratlys to a negotiating table as representatives of Republic of China military administration.
11. On the morning of day x of riots on all Taiwanese islands a short clip is sent to all the media outlets. It shows the PLA officer shaking hands with the Republic of China officer and beginning negotiations on the possible political transition of the Taiping Island, Republic of China to the People's Republic of China under "once country, two systems" model. This time when the Taiwanese ships arrive at the Taiping islands they are met by PLAN vessels who refuse to let them through as under the arrangement between the interim military administration and the PLA only the KMT has the authority to negotiate with CPC with regards to the Taiping island. The government in Beijing formally invites the leader of KMT to negotiations in Beijing.
It is exaggerated and far-fetched and possibly it ignores certain details about the specific islands as well as the complexities of this senario in a broader context - including intervention of external actors and the timescale that might work to the advantage or one side or the other. It is not intended to be perfect. What I wanted to demonstrate is how such a scenario can unfold without direct military intervention from the PLA. The goal is to present DPP as incompetent and unable to control the state in a manner that is conducive to the three pillars of authority - security, stability and prosperity. Ultimately it is intended to put Taiwan in a state of civil war with DPP as the government being challenged by the people, even if they are a minority. If you can undermine the "self-determination, democracy and human rights" narative of DPP by forcing them to violate those principles - you have won. They've lost the mandate of heaven.
And boy oh boy if the US and Japanese military intervention in support of DPP's effort to repress the protests and insurgency is not best possible solution. A propaganda coup like no other. It would only delay the inevitable but completely ruin America's image worldwide.
The key here is the ability to resist information onslaught from the US which will require about a decade of work. This is the gap between the capabilities of US and China in the cyber domain according to London-based International Institute for Strategic Studies. So it is not about navies and air forces but about who gets to spin the story better. China has the advantage locally but not globally. If they can match the US globally then DPP is toast because the US will not sacifice its grip on all the other assets worldwide (Europe, South America, Middle East, India) to save Taiwan - and that's what would happen if they were to go head to head with China. They would likely still lose Taiwan but in the process they'd lose control in numerous other places. Just think how much damage one Trump did without anyone contending with the US information warfare directly. This would be far worse.