Once again, the self-fulfilling prophecy. Saying this dissuades third party voters, which causes the two party system to be upheld.
The history of the United States is that we have essentially had a two party system since 1852 (President) and 1856 (Congress), with smatterings of others which never got off the ground sufficiently to do more than perhaps be spoilers.
However, I suspect that moderate Democrats might agree with me that what we are experiencing now, with the far left radicalism being exhibited by many under 40 and the “leadership” of a minority in Congress, and the apparent buy-in of many governors and large city mayors and council members, that we may have the making of a three party system.
Should Trump win, and particularly if he wins by more than a slim majority, we may see a rethinking of the Democratic party with enough who want to disavow the radicalism to dissociate from the radicals and the Marxists and form a centrist party. It would take a whole lot of work to be able to do so, as 160+ years of status quo is hard to work against.
Currently, Democrats outnumber Republicans as far as registration goes. And as far as drawing much from the list of 3rd party member, it is unlikely they would draw enough to sustain a viable party; they would need to draw from Republicans.
One could envision a break-off from the Democratic Party which is pro life (thus able to draw both Catholics and evangelicals) and pro good policing (another whole topic), pro small business and possibly pro labor (ignored by Hillary and the Democratic Party as shown by the last election). But to be large enough to not be lost in the war of current politics it would need to take note of policies and achievements of the current administration, which has been led by a President who is more a populist than a dyed in the wool Republican. In short, they would have to draw from the best of both parties, and draw enough that they could get past a divided Left and not be steam rolled by a united Right.
In the 1960’s, while in high school I talked with my grandfather who was a staunch Democrat. He was a small business owner mechanic who worked on cars, trucks and farm machinery, and his point of view was that big business would destroy small businesses - his example then being Safeway driving out all small grocers. I for years was a registered Democrat, but have never voted for anyone pro choice/pro abortion - which left me with precious few Democrats to vote for.