How and by who? He was pretty well entrenched already though not yet supported by various fresh organisations like Night Watch.
Indeed who is doing the taking down. Was it Earthforce (General Haig)?
I'd say that would be problematic, to put it mildly. There was little to no concrete proof he was beihind Santiago's death. IIRC it wasn't even clear for a while Santigo was assassianted.
IIRC There was pretty good evidence to link Clark to the assassination. Clark was recorded on an EarthForce high security communications channel admitting he wanted Santiago dead and stating "I wasn't sure we could pull it off." After which an unidentified voice (Morden) told Clark that Earth Force 1 would never return from Io. Per Sheridan those com signals carry identifier codes on a sub-channel which cannot be faked, if he's right that is pretty powerful evidence there. There were also medical records that show Clark was not sick prior to getting off EarthForce 1 (contrary to his claims). And Garibaldi would be able to testify to finding the jamming equipment on Babylon 5 that was set to jam EarthForce One's com systems, after which he was shot by one of his subordinates (who then mysteriously escaped from EarthForce custody after being personally ordered back from Babylon 5 by President Clark.) That should be sufficient evidence at least for the Earth Alliance Senate to impeach Clark and have him removed from office.
What gave Clark the perfect excuse to declare martial law before he was impeached ironically, was when that shadow vessel destroyed the research center on that Moon of Jupiter and was blamed on Sheridan and Delenn in the white star.
With the passing of years, however, it's become a more dubious plot twist for me. Yes, one could talk about the trauma of the Earth-Minbari War increasing people's anti-alien paranoia and strengthening the support for fascist organizations like the Night Watch and such, Clark having the help of the Psi Corps and the Shadows to enable him to put loyal personnel to key positions in the military etc., but it still doesn't say much about the strength of the Earth Alliance's checks and balances and democracy in general that it could all be subverted in just a very few years.
What I mean is that Clark declaring martial law wasn't anymore a case of going after a few malcontents and cranks; he sent troops to storm the Earth Senate, the legislative body of EarthGov itself. That would be like Nixon sending in the army to storm Congress during the height of Watergate.
I remember at the time I was stoked by the plot development, how in one stroke Earth became a dictatorship.
With the passing of years, however, it's become a more dubious plot twist for me. Yes, one could talk about the trauma of the Earth-Minbari War increasing people's anti-alien paranoia and strengthening the support for fascist organizations like the Night Watch and such, Clark having the help of the Psi Corps and the Shadows to enable him to put loyal personnel to key positions in the military etc., but it still doesn't say much about the strength of the Earth Alliance's checks and balances and democracy in general that it could all be subverted in just a very few years.
What I mean is that Clark declaring martial law wasn't anymore a case of going after a few malcontents and cranks; he sent troops to storm the Earth Senate, the legislative body of EarthGov itself. That would be like Nixon sending in the army to storm Congress during the height of Watergate.
On the other hand there were hints even before Clark came to power that the Earth Alliance was deeply dysfunctional. An episode in the first season wanted us to take seriously the idea that the Earth Alliance might actually shoot striking workers (and implied the EA had done so in recent memory.) And Earth was also holding down Mars by military force while managing the planet so poorly that the Martians had rioted for food within the last few years. So the EA seemed pretty illiberal even before Clark took power.
A more plausible story might have been if a nuclear bomb had gone off in EarthDome (at a time when Clark just happened to be out of the capital of course.) The EA Senate and Supreme Court get wiped out in one blow, and the bombing gives Clark a crisis big enough that martial law (only for the duration of the emergency of course) would seem justified to most of the EA population. (Especially if EA investigators just happen to find "proof" that the nuclear bomb was placed there by the Minbari as part of a plot to decapitate the Earth government in preparation for an invasion.) Clark then has the EA go to war with the Minbari (assuming his Shadows allies will help him win it of course), and the EA is pretty much his. (Which also has the story benefit of making Earth an actual part of the Shadow War.)