What would it take for Russia to launch a first strike?
Probably not nearly as much as it would take for the US Congress to proceed with imposing the rule of law on the Current Regime now squatting in the White Hut.
Putin, and Russia as a whole, see the governance of that territory once known as "the United States of America" as swimming in chaos and indecision. The Russians would consider it a act of mercy on their part if the Current Regime were taken down and humiliated sufficiently before the world so the Current Regime would resign and flee.
Cresson Kearny mentioned in Nuclear War Survival Skills, 1989 edition, that a Russian first strike could easily be done with a submarine missile salvo. Russian ballistic missile subs could get a decent distance from the coast before firing. The missiles would then detonate at orbital altitudes, triggering EMP. The detonations would also trigger interference, making us blind in our detection systems. Durin the blackout, additional attacks would occur ranging from bombers to ICBMs, to sinking key naval vessels such as carriers. The first priority in nuclear war is to destroy the means of the enemy to use nuclear weapons against you. The Second is to cripple any military. Te third is to destroy te other side’s government so that the population will die out of chaos an panic.