One good guideline in thinking about it is that if it's illegal, that means it's officially improper.
Most contact between Americans and non-Americans are perfectly legal. From business deals to personal contacts to hearing presentations made by foreign officials to sports competitions to attending shows by foreign celebrities to being a tourist in another country, most of it is far from being illegal or improper in any meaningful sense. Fiona Hill, the top Russia adviser on the National Security Council, who I've quoted here recently, has dual UK-US citizenship. It's legal for her to associate with herself. And it doesn't prevent her from getting a security clearance.
There are American laws regulating American business dealings with foreign firms. That's why companies have lawyers.
And there are laws about foreigners contributing financially to American political campaigns. There are also laws and regulations about governments directly influencing political campaigns. And laws against hacking other people's computers. And working with foreign intelligence agencies. The Washington Post has a column by a former CIA officer, Trump Jr.’s Russia meeting sure sounds like a Russian intelligence operation 07/14/2017, that explains why the now-notorious Don Jr./Veselnitskaya meeting looks so much like dirty business. Although that's already pretty obvious to everyone but partisan Republicans who will defend anything Trump does.
Obviously, political ideas don't stop at national borders. Nor should they. And there is a broad spectrum of things that facilitate those. There are books and movies and TV reporting and commentary, for instance. At the other end of the continuum we would have paid agents promoting political subversion or, say, hacking the computers of one party or campaign to give an advantage to another. Hacking into election systems is also illegal, even if they don't do it to manipulate the vote.
It's impossible to not thing about precedents in this situation. There were Nazi Fifth Column operations to deal with in the 1930s and 1940s. And there the Red Scares after both world wars, the one after the Second World War being largely remembered as McCarthyism.
There were several sensational cases of espionage in the postwar era involving Communists. In the United States, the conviction and exection of Julius and Ethel Rosenberg for passing atomic secrets to the Soviet Union was easily the most serious. Klaus Fuchs was a german-born British Communist who also passed atomic secrets to the USSR. He was imprisoned for several years in Britain, but later was released to East Germany, where he became deputy director of the East German Central Institute for Nuclear Research. Marian Smith Holmes has an article about atomic espionage figures, Spies Who Spilled Atomic Bomb Secrets Smithsonian 04/19/2009.
Alger Hiss became (in)famous as a target of investigation by California Congressman Richard Nixon and the House Un-American Activities Committee (HUAC). He was convicted not of espionage but of perjuring himself before Congress by denying he had been a Soviet agent. There are respectable historians who believe that Hiss was guilty, and that seems to be the leading opinion. Hiss himself denied to the end of his life that he had been a Soviet agent. And there is still no clear historical consensus on Hiss' guilt. The loathsome nature of Hiss' main accuser, Whittaker Chambers, and the role of the perennially unsavory Tricky Dicky in his case are more than sufficient cause to be reserved about the conclusiveness of the case.
Robert Oppenheimer, civilian head of the Los Alamos laboratory in the Manhattan Project, had his security clearance removed and was accused of espionage for the Soviets, as well. Hardcore rightwingers still like to make that claim. But it remains a far-fetched one.
But if an American political party wants to take a foreign government or movement as in some way a model to which to aspire, there's nothing illegal about that in itself. In the case of the US Communist Party, they explicitly took the Soviet Union as a model to emulate. There's some argument to be made that this association helped them win support after the Great Depression struck capitalist country but not the USSR. And, of course, the Soviets were American allies in the war, which also helped the US party's image somewhat. That didn't mean that the armed services didn't look askance at known or suspected Communists in the armed forces. Oppenheimer's admitted association with Communists, though he always claimed he was never actually a party member, caused him problems after the war. But the Army was willing to put enough confidence in him that he had a key role in developing the atomic bomb.
But the Communist Party's close identification with the Soviet Union was probably more of a turnoff to potential supporters most of the time than a net benefit. Because taking a dim view of Russia has been more the norm than the exception for Americans during the last century or more. The Czar wasn't that popular for Americans, either, though Russia was also an American ally in the First World War.
Speaking of tradition, the way some commentators and politicians talk about the Trump-Russia scandal has notable echoes of the way anti-Communists talked about Communists and the Soviet Union. And conservatives like Nixon and hardcore rightwingers like Joe McCarthy were more than happy to identify liberals and Democrats with Communists and the USSR.
I was reminded of one comparison to today's situation reading this, "Nothing so effectively shores up the American predilection for moral certainty as a good batch of incriminating documents." (Maurice Isserman and Ellen Schrecker, "'Papers of a Dangerous Tendency': From Major Andre's Boot to the VENONA Files"; Cold War Triumphalism: The Misuse of History After the Fall of Communism, E. Schrecker, ed.; 2004)
Whittaker Chambers' Pumpkin Papers stunt is a good example of using "incriminating documents" to good effect.
But the (digital) papers that Don Jr. released about his Veselnitskaya meeting are more damning than anything in the Pumpkin Papers were for Alger Hiss!
Isserman and Schrecker have some other worthwhile observations based on information including a number of Soviet documents release after the fall of the USSR:
Of the approximately 50,000 party members in World War II, 49,700 were uninvolved in espionage, even taking the highest estimate of communist participation in the KGB's network. The average Communist in 1944 was far more likely tobe a fur worker or a public school teacher than a policy maker in the Treasury Department, and thus an unlikely candidate for a Soviet operative to approach for workplace gossip. And even among the tiny minority of Communists and their fellow travelers who did occupy sensitive posts, not all were necessarily approached or agreed to spy on behalf of the Soviet Union.After the Second World War, the Soviets tended to recruit agents in more conventional ways rather than on an ideological basis. Aldrich Ames, arrested in 1994 on espionage charges, was a CIA employee. The Senate Select Committee on Intelligence reported in 1994 (An Assessment of the Aldrich H. Ames Espionage Case and Its Implications for U.S. Intelligence 11/01/1994):
While the precise extent of Ames's espionage activities was unclear at the time of his arrest, Justice Department officials confirmed that Ames was believed to have caused the death or imprisonment of a number of Soviets who had been sources of the CIA and FBI. ...The blowhard alcoholic Joe McCarthy who claimed he had a list of Communist employees of the State Department never uncovered a single Communist in federal employment, much less a Russian spy. Not even with the help of his lieutenant, the future mob lawyer and political mentor to Donald Trump, Roy Cohn. Isserman and Schrecker write that McCarthy "was not simply 'exaggerating' a problem - he was making it up."
The affidavit made public at the time of the arrests also confirmed that Ames had received substantial payments for the information he had provided -- money that he had used years earlier to purchase a new Jaguar automobile and a $540,000 home, with cash, in Arlington. Apparently, these seemingly large expenditures by an employee making less than $70,000 a year had not raised questions at the CIA.
On April 28, 1994, Ames and his wife, Rosario, pled guilty to charges stemming from their espionage activities. Entered into the record at the time the pleas were made was an agreed-upon "Statement of Facts" which provided new details regarding the Ames's espionage activities. Meetings with the Soviets in Washington, D.C., Vienna, Bogota, and Caracas were acknowledged for the first time. Ames also acknowledged that as of May 1, 1989, he had been paid over $1.8 million by the KGB and that $900,000 more had been set aside for him.
They also note, "We also now know that by 1953, the year of the Rosenbergs' execution, the FBI had quietly written off the American Communist Party as a serious espionage threat." Go figure. It's a reminder to everyone to keep their critical thinking skills working in the Trump-Russia scandal.
They also quote Richard Hofstadter, "[T]here are conspiratorial acts in history "and there is nothing paranoid in taking note of them."