I would like to point out that Hoover never officially blackmailed anyone.
While there were persistent rumors, such as President Truman's comments that the FBI was trending towards the Gestapo and that Hoover was "dabbling in sex-life scandals and plain blackmail." The official unofficial explanation was that Hoover kept confidential files segregated in order to keep file clerks from gossiping about celebrities or politicians.
Unofficially... "We found out about X. We promise to tell no one." By the letter of the law, it probably wasn't legally blackmail. By common sense and normal person's perspective, it was obvious coercion, if not blackmail. That was Hoover's game. Letting some percentage of politicians directly know he had dirt on them or their family, but not directly threatening them.
The NSA is currently playing a different game. They're generally NOT letting anyone know what dirt they may or may not have. They mostly bribe their way with the telecommunication companies. NSA's Special Source Operations pays out hundreds of millions per year for consumer data. This is only about $300m-$400m. They pay communication companies for legitimate services, which is in the tens of billions per year in leased lines, services and whatnot. Some through DISA (only a couple billion), some through the individual services like NMCI (another single digit billions), some through other intelligence activities (unknown) and some directly(unknown). I have no idea how much official or unofficial pull they have with telecommunication procurement with the rest of the federal government.
They generally do not directly say "If you want those billions of dollars, you better let us have access to your wires and not ask questions."
Most companies look those billions and take the money. They legally cannot be prosecuted or sued, as they have been given immunity from wiretapping laws.
Only a complete moron like Nacchio bucked those easy billions. He thought he could tell the NSA to shove the illegal wiretapping but keep the lucrative natural security contracts. He was sentenced to six years in prison. The circumstances relating to the NSA and billions in federal contracts were classified and ruled inadmissible in court. Kinda hard to win a court case when the prosecutors can keep out a huge chunk of information proving your innocence. Not that I personally think Nacchio was squeaky clean, mind you. Also, I'm not sure if it's illegal for the NSA to notify SEC of a CEO's insider trading. The NSA's activities were illegal at the time. Most of their domestic wiretapping has been legalized, though it remains unconstitutional.
Now, you might be saying "Uh, that's a nice rant ya got there. But what does it have to do with blackmailing politicians?"
Fair enough question.
The NSA has been spying on domestic communication since 1978 under BLARNEY. Prior to BLARNEY, it was SHAMROCK and MINARET (started in 1945).
Originally SHAMROCK and MINARET were designed to spy on communists, communications to communists, potential communists or pro Civil Rights persons. Then it moved towards anyone against the Vietnam War. Since then, it's spread to more widespread mass surveillance.
Current partially or wholly domestic projects are:
DISHFIRE - Mass collection of text messages (cellular)
MUSCULAR - Tapping data transmitted between major corporate data centers (Google, Yahoo, Microsoft, etc)
Turbulence - Collection of programs. Malware, poisoned routers, cable taps, etc
PRISM - Tapping internet traffic
Main Core - List of 8 million American suspected of being potential threats
MYSTIC - Voice interception of entire country. Previous targets Iraq, Afghanistan, Bahamas, etc.
MAINWAY - Metadata warehousing
RAGTIME - Domestic counterterrorism information obtained under FISC (ie probably legal)
APS Users : "So, you have an extensive and long list of domestic surveillance. And?"
Exactly zero of those programs have specific filters to remove politicians from their take. The NSA publicly said so in their response to an inquiry from Senator Bernie Sanders (I-VT). Congress receives no special protections or different treatment.
The NSA does have a history of both intentionally and unintentionally spying on Congress. During the Vietnam era, they spied on anti-war politicians. During the Church Commission, they spied on Frank Church. During the CIA torture inquiry, they spied on politicians involved. Does the NSA spy on Congress? Yes, directly and indirectly.
So, no, as far as anyone knows or can prove, Congress is not being blackmailed, in the legal sense.
As far as anyone knows or can prove, the NSA is not pulling "soft" blackmail or coercion in the style of Hoover.
So, they have the potential means of doing so, but as far as anyone knows, they are not employing it. Per my earlier statement, I believe my "capability is the blackmail" statement is correct. HOWEVER, I wish to clarify that I mean this in the common sense meaning of the word rather than federal law. Coercion is a better term if you want to use legal terms. I believe blanket, unwarranted and unaccountable surveillance of both the Legislative and Judicial branches of the Federal government gives the intelligence community leverage that they should not be given. While it is (probably) legal under existing law, I believe this to be unconstitutional under the Fourth Amendment.