The US Constitution does not have loopholes. The “runaway convention” is a myth propagated by conspiracy theorists and those who want an all-powerful centralized government.
The state legislatures will select delegates they trust to carry out their wishes at the Convention for proposing Amendments. The Convention for proposing Amendments has, under the US Constitution, the limited power only to propose amendments. Any proposal coming out of the Convention for proposing Amendments must be ratified by three-fourths of the states.
Moreover, Conventions don’t runaway. There have been hundreds of state constitutional conventions and none have runaway.
The “Runaway Convention” is a myth.
Finally, trying to envision a runaway convention scenario reveals its weaknesses. Let’s say we reach the 34 states applying for the amendment to repeal Obamacare and Congress calls a convention for proposing amendments. The state legislatures would then send delegates to hammer out the exact language. Now, of course the delegates from liberal states may attempt to hijack the convention for proposing amendments, but since the majority (at least 2/3) are there to propose an amendment repealing Obamacare, this could not occur. But the runaway myth would have us believe that once at the convention, the delegates appointed by the states supporting the repeal of Obamacare will simply change their minds and decide to rewrite the constitution or propose some left wing amendment like repealing the Second Amendment. Then, the Legislatures who originally just wanted Obamacare repealed would then abandon the effort to repeal Obamacare and sign on to the left wing amendment. This is simply unrealistic fantasy.
Wikipedia actually has a nice summary of the Convention to propose amendments to the United States Constitution:
And here’s a scholarly examination of the state led process from Harvard law: