The global political consequences of nuclear testing are too dire for any non-rogue regime to consider. I would argue that only America has the ability to justify a return to testing if the Russians test first. Even that would be hugely problematic, as the Indians are still very angry about how they were sanctioned in the late 1990s. The fallout of those sanctions continues to the present day. If Clinton had simply ignored the 1998 tests, the United States would have replaced Russia as India’s main defense supplier and India would have been firmly in our camp regarding the current ongoing conflict, not neutral or ambivalent.
A return to testing also might signal the beginning of an unprecedented proliferation. The South Koreans have been very publicly debating the issue, which is perfectly acceptable, as long as it’s just a debate. US testing would likely be interpreted as tacit approval for a South Korean program and for Taiwan and Japan as well. After that, you might as well throw away the entire NPT. I could name every country that could afford a deterrent and might pursue one. It’s quite a long list.