In a letter released Sunday, a group of Senate Democrats demanded that President Trump provide "tangible, verifiable progress on denuclearization" from his second summit this week with North Korean leader Kim Jong-un in Vietnam.

Details: The eight lawmakers, including Senate Minority Leader Chuck Schumer, acknowledged that Kim has shown a "change in behavior" since the two leaders first met in Singapore last year, but said the isolated country still poses a nuclear threat and argued that Trump should "execute a serious diplomatic plan," including a process to verifiably freeze and roll back North Korea's nuclear weapons and missiles programs.

Go deeper: Trump team unsure if North Korea is serious about denuclearization