Was Alexander Haig the 'de facto' President in 1974?

[DevotedDemocrat]

I've read at times, after the resignations of Haldeman and Ehlichman in April 1973, and with the Watergate Scandal increasingly consuming both his time and credibility, President Nixon began to actually lose his power as President, and that de facto control of the country fell to Alexander Haig (in terms of domestic policy) and Henry Kissinger (in terms of foreign policy) in the last six months or so of his Presidency--around the Spring/Summer of 1974.

I've read some accounts that go so far as to paint Haig as the "de facto" President in the last months of Nixon's term.

How true is this?
If so, when did Nixon's grip on power become so weak that Haig and Kissinger began to run the country for him--at what point can we pin point it?

If not, was Nixon still in command, in control, in the last months (say beginning January 1974)?

And why, in most accounts, are the resignations of Haldeman and Ehlichman portrayed as being incredibly devastating to Nixon's Presidency?