The US gradually got drawn into Vietnam over the course of a decade or so but LBJ made the key decision to escalate US presence in 1965. The Vietnam war turned out to be perhaps the single most important political issue in the US between 1945 - 2001. It bitterly divided the democratic party against itself, the left against the right in America and fueled political extremism as well as setting the battle lines between red America and blue America from the late 60s all the way up to today. The rise of Nixon, Reagan, Trump and the marginalization of the Democratic party from the 70s to the 2000s all had its roots in the experience of Vietnam.
What if LBJ decides not to americanize the war in 1965 and just keeps advisors and maybe some marines to guard airfields? Let's assume (and this is a big one) that furthermore the Johnson administration does not commit itself to a "Vietnam-equivalent" in another part of the world. Let's say this isn't enough to keep the South Vietnamese government propped up and Saigon falls sometime in the late 60s. Let's say Communism spreads to Cambodia and Laos as per OTL but ultimately the status of Vietnam within the context of Sino-Soviet rivalry leads to something like the OTL power struggle between the PRC and USSR in SE Asia pitting Cambodia against Vietnam. Something like the 1979 Chinese invasion of Vietnam still happens but earlier, in general though let's assume the events in the region simply gets moved up 5-7 years but mostly goes the same as OTL: Deng still comes to power in China and Brezhnev stays in power in the USSR.
How does this affect US domestic politics: does LBJ run again in 1968 (his health was pretty bad and his popularity might take a dip for being "soft on communism") do the democrats still split in 1968? Does LBJ push more social programs on the heels of medicare/medicaid? Is McGovern and his young supporters and Carter still the face of the Democratic party in the 70s or does the new deal coalition stay intact? Do the Republicans still run "law and order" and succeed with the southern strategy? Does Reagan conservatism still take over the GOP by the 80s?