Or like when in 2000, the election of George Bush the Younger was aided by the third party Green candidate Ralph Nader, who siphoned off enough votes from Democrats for Republican Bush to win by a little over 500 votes, thus taking the winner take all state of Florida, https://uselectionatlas.org/RESULTS/national.php?year=2000. In our more modern era, third parties have been nothing more than spoilers for deciding who wins the national election between Democrats and Republicans. So when an outreach is forming from the left to go third party the Republicans will ultimately benefit. Same goes from an outreach from the right, Democrats benefit.
So it would behoove our Democratic party to work within the Democratic party to change our policies to reflect our more modern vision. Any other course of action to go third party is a direct slap in the face of us Democrats who are trying to change our party from within. The cost of such a third party attempt by Democrats brings about a benefit to Republicans like the current trump dilemma. We have to get our base voters out and turn Independents toward our message regardless if our message is perfect for us or not. Remember, we are building a consensus of a big tent strategy which means we must attract others who are not as pure liberal/progressives as we are but who at least share a general sense that we are working for not only ourselves but for them as well.