3 times

At least three times in my life has a Republican presidential candidate conspired with a foreign power in order to attain the office of the Presidency and succeeded.
At least twice, the previous Democratic administrations had been aware but decided to not bring the case to law enforcement or to the American people for fear that the press would consider it "politicizing" or fear of burning intelligence assets.

• In 1968 Anna Chennault, conspired with the South Vietnamese government to scuttle the Paris peace talks, assuring Saigon that Nixon would give them a much better deal if he became President. It was a clear violation of the Logan act, which Johnson called treason. Chennault was directed by Nixon to contact Saigon and keep the Paris peace process from continuing. 

• In 1980, the Reagan team clearly executed an agreement with Tehran to keep the American hostages until after he won the election, preventing Carter from getting an October Surprise. Yeah, the Gary Sick allegations turned out to be nonsense, but let's get real -- the hostages were turned over 20 minutes after Reagan was inaugurated and then, for some reason, the Reagan Administration decided to participate in the Iran-Contra Affair. 

• And now, Trump/Putin.