Richard Nixon committed treason during the 1968 election campaign by interfering with the Paris peace talks to end the Vietnam War. He got away with it because LBJ was reluctant to publicize the treason. Nixon was forced to resign in 1974 because he lied about the Watergate break-in in 1972, designed to help him defeat George McGovern in 1972. The irony was that he won so overwhelming that he did not need the Watergate effort to win.
Ronald Reagan’s team interfered with the Iranian hostage negotiations during the 1980 election campaign. He should have been impeached, but he was not. Reagan averted impeachment again during the Iran-Contra affair in 1986. He claimed he could not remember, thus averting the need to lie. Presidents are impeached for lying, not treason, witness Clinton’s lies about Monica Lewinsky.
It should be easy to impeach Donald Trump. He lies all the time. Just ask him a significant question, and he will lie. Done deal.