In the efforts to block the nomination of John Bolton as Ambassador to the UN much has been made of his personal behavior, his reputed temper, his dealings with underlings and his brash manner. His actual beliefs and positions while at the State Department have been brought up, but some issues on which he's been very outspoken have been studiously, even deliberately avoided. Right now the main sticking point seems to be Bolton's unwillingness to release drafts of research he did on WMDs in Syria - documents which have nothing to do with his appointment, and which Senators could request through regular channels rather than using the confirmation hearings as a fishing expedition.
What Bolton's opponents are dancing around, but may actually be their main objection to his nomination, is his very strong opposition to gun control and international interference in the US legal system. While at the State Department, Bolton was instrumental in blocking efforts at the UN to pass sweeping gun control mandates which would have attempted to override the 2nd Amendment of the US Constitution. He was also very active in blocking efforts to keep the International Criminal Court from having even highly limited jurisdiction within the US. Given these and similar positions opposing international interference in US sovereignty, it's not surprising that the most outspoken internationalists and the biggest anti-gun figures in the Senate are among those working hardest to oppose Bolton.
One of Bolton's most outspoken opponents is Senator Chris Dodd (D-CT), who is on record as having voted for every gun control measure ever put before him as well as voting in favor of eliminating sanctions against Cuba, Vietnam and China, supporting the IMF and the World Court, and voting against preserving US sovereignty and for internationalist interests at every opportunity. Dodd is essentially spearheading the opposition to Bolton, and he's more anti-gun and pro UN than anyone in the Senate. He has help, of course, from folks like anti-gun extremist Diane Feinstein (D-CA) and fellow internationalist and gun-hater Charles Shumer (D-NY). Now, you might argue that all the Democrats are against guns and for surrendering US sovereignty to the UN and international treaty organizations, but the key fact is that other Democrats are mostly just along for the ride, it's the really rabid internationalists who are leading the opposition.