Considering that dozens of consistently changing positions in the US Senate and congress, has anyone one of their holders ever turned against the US through things like foreign bribes or outright betrayal? Whats the closest instance?
Yes. In 1798 Tennessee Senator William Blount conspired to give control of Louisiana and Florida to the British. He owned a lot of land in the southwest which had lost significant value as a result of a fear that the French would gain control of Louisiana and cut off access to the Mississippi. The conspiracy was an attempt to give control of Louisiana to England with guaranteed access to the Mississippi for the U.S., and in turn, drive the land prices back up in the southwest. Instead, one of his fellow conspiritors turned in a letter written by Blount outlining the plan to a government agent. The house immediately voted to proceed with impeachment hearings, but not before the Senate essentially expelled him by voting to sequester his seat. The hearings never went anywhere and were eventually dismissed. He went back to Tennessee, and died a couple of years later. To my knowledge, he never actually faced any formal punishment other than a badly marred reputation. Oddly, because he was a was an original signer of the Constitution, there is now a statue of him in Philadelphia at the Signer's Hall.
Congressman Samuel Dickstein was a Congressman from New York who was shown, according to papers found in Russian archives in the 1990s, to have accepted payment from the NKVD (predecessor agency to the KGB) in the 1930s. According to those Russian sources, Dickstein was basically a run-of-the-mill criminal and was turning over secrets mainly for money, although Dickstein claimed to his Russian handlers otherwise. I'm not certain that anything Dickstein turned over was classified, but he certainly worked as a Soviet agent. It's also worth noting that those Russian sources also mention that Dickstein admitted to them (i.e. the Soviets) that he had previously spied for both British and Polish intelligence.
None of this was known until Russian archives opened, so long after Dickstein died. As an aside, there is still a street (or was in 2013 anyway) named for Dickstein in in the Lower East Side of Manhattan.
You might be interested in this thread about Richard Nixon regarding his actions at the end of the Vietnam war
I have a follow up question: presumably no security clearance is required to be a senator or congressman since it is a fairly fundamental right to run for public office. Do politicians on the various national security committees who have access to highly sensitive information require top secret clearances the same as any civilian or military person?
Has there ever been a historical determination regarding allegations about a KGB memo stating Ted Kennedy wanted to work with them in exchange for their help in defeating Reagan electorally?
Abscam was an FBI sting operation started in 1978. The original target was members of organized crime rings, but the front operation was very successful (the FBI posed as liaisons for a wealthy Arab sheikh), and the penetration led the agency toward contacts in the political realm.
After handing out $400k in bribes, the FBI found several politicians willing to grant asylum to the sheikh in exchange for cash. Six congressmen and a senator were convicted.
I do believe that in a round about way information was passed on to British agents prior to World War 2. The British sent agents specifically to socialize with American high society for a twofold purpose: 1) Gather information about American leanings towards the British cause 2) influence the wives of government officials to advocate on behalf of the British to their husbands. The agents were encouraged to use any means necessary, including sleeping with their wives. While the book never mentions specifically what information was given to the British, it does say a significant amount of classified information was passed to the British, all regarding the governments stance on the war.
Source: For The President's Eyes Only
Vice President Jefferson apparently undermined U.S. diplomacy, and urged the French to wait for the next administration. here's two books that have a quote