The Dems are doing much more than 'gritting their teeth'. It would not surprise me at all to find out that they are colluding with South Korea, China, Taiwan, Europe, Iran, and anyone else they can drag into it, to either have the summit called off, or at least to shift the credit to China or South Korea or literally anyone else other than Trump.
I am sure from their words and actions that the Dems would much rather see a war in North Korea during Trump's term, than they would see Peace. The Dems will do anything/everything they can possibly do to muck this up for Trump. Maintaining their power, their facade of greatness, means much more to them than human life or world peace. Even if the deaths would be in the tens of millions.