There is no real evidence to support this claim. Where the constitution was intended to require greater than a simple majority, it is spelled out specifically. That was not the case for admission of new states.
Although there are many other good and legitimate reasons for PR Statehood, no reason to deny that the fact that they’d likely vote Democratic for the foreseeable future is one of them. In that spirit, maybe we should consider splitting it up; Puerto Rico has enough people to be divided into FOUR States that would each be bigger than the smallest current State. Let’s think big here; why settle for two new Dem Senators when we could have eight?!
But in the most recent referendum, statehood already got over 50%. Votes between three or more options can get tricky, because it’s possible for none of the three options to get a clear majority. That’s where things like dropping the lowest and having a run-off come in. But that’s not necessary here. If we dropped the lowest of the three options and re-did the referendum, we already know which one would win, because statehood would still get at least 50%. And probably well over the 52% that Newfoundland had, because at least some of the folks who voted for the lowest option (I’m not sure offhand which that was) would switch to statehood.
OK that is a fun fantasy exercise but at the same time I’ll step in with my PRican-from-PRico hat and say that is not something to rest upon. We have alternated electing R-aligned governors/congressional delegates and conservative legislative majorities for years.
Oh, I hadn’t realized that. Guess it’s back to pushing my “Statehood for San Francisco” plan. 
Massachusetts has in the recent past [Romney] and does now have a Republican governor. However, those Republicans are not like the Republicans in Texas, for example. I suspect that the same is true or Puerto Rico.
The modern day Massachusetts Republican elected official is often a combination of a few things, because Dems are basically so dominant in the State, voting for say, a Republican Governor can be a way to express dissatisfaction with status quo from the Dems. However, for that to occur the candidate typically needs to be very moderate, moderate enough that in many non-Massachusetts states, they would potentially just be Democrats anyway. Romney largely fit that description in the era in which he was Massachusetts Governor, as does Charlie Baker now.
Romney found out it is difficult to transition from that position to national GOP politics–in his 2008 primary against McCain, he consistently struggled to be seen as a “real” Republican, because he had come out forcefully as pro-choice, in favor of a form of universal healthcare etc–i.e. things that are disqualifying in the national party. He spent several years burnishing his right wing bonafides to essentially quell concerns about those apostasies, and did so sufficiently well to secure the nomination in 2012.
Romney had the bad fortune to be trying to move gently rightwards from slightly left of center at a time the party was veering hard rightwards at a headlong gallop. The fastest he could move still left him ever more on the left wing of his party. The then-vestigial and now non-existent left wing.
Many of the current questions on Puerto Rican statehood were common with New Mexico - and to a lesser degree Arizona - during the late nineteenth century.
Ultimately, New Mexico’s case for statehood in 1912 involved a campaign to cast its large Hispano population as “Spanish American” - and therefore white by law - as opposed to Mestizo/Mexican. The growth of the “Anglo” population was also a factor there.
One issue that makes it complicated is that while I sense most “mainstream Liberals” in the Democratic Party strongly support Puerto Rican statehood, there is a strong element of the “Progressive Left” that views the U.S. relationship with Puerto Rico as deeply colonial. Making Puerto Rico a state would only cement colonial rule. For instance, Luis Gutiérrez went ballistic when John Ossoff expressed support for Puerto Rican statehood in his 2020 senate campaign.
Puerto Rican voters might not be as solidly Democratic as imagined, and statehood might reshape political identity in unpredictable ways.