hi all! Just a reminder to contain all discussion in this thread to events prior to 1995 only, per this subreddit's 20-year rule. Thx!

The most famous example would have to be Congress refusing to join the League of Nations after Woodrow Wilson was instrumental in establishing the organization.

The main one I've been able to find that follows the 20-year rule on this subreddit is Senator Ted Kennedy's letter to Yuri Andropov concerning undermining Reagan's reelection efforts. Unfortunately, the archives are apparently sealed, and I'm only able to find excerpts from potentially biased sources, so hopefully somebody else with better research access can find the full document or use it as a starting point for a better discussion.

In 1975, Senators John Sparkman and George McGovern were accused of violating the Logan Act when they traveled to Cuba and met with officials there. It was decided not to press charges back then. It is hard to know for sure if their discussions "undermined a President's diplomatic efforts", since the contents of their discussions were never made public.