That's been true for a long time, at least insofar as political parties have clout. But the Duopoly is entrenched and bends over backward to make sure it stays entrenched, right down to controlling the Presidential debates which if anyone doesn't know are under the strict control of the Demoplican-Republicrat Party -- which means no questions, and no candidates, will show up in them without the approval of the Duopoly.
How much racketeering is THAT.
And their most useful tool in keeping any third party shut out is the Electoral College with its WTA scheme. That's why the vast majority of us vote not FOR one candidate but to BLOCK another, it's why we've been given the perpetual choice of "Bad" or "Worse". In 2016 three states famously swung the outcome, Michigan, Wisconsin and Pennsylvania. NONE of those states voted in the majority for the candidate that got ALL of their electoral votes. And that's a racket.
Gore Vidal famously noted, "as for a third party, we tried that in 1972, the People's Party. Unfortunately we hadn't remembered that in order to have a
third party you must first have
two other parties". We haven't had that for over a century.