I'd be surprised if any US leftist other than Chuck0-like anarchists (abstention), Solidarity and the ISO (a third party), and incorrigible pacifists (Dennis Kucinich), who together constitute a small minority of US leftists, would campaign for anyone other than a Master-Class-approved Democratic Party candidate and try to change the parameters of "electability" (the seal of Master Class approval, signified by direct and indirect campaign donations, corporate media coverage, etc.).
Being on the Left in the USA basically boils down to being for the Master Class's Democratic Party (especially for offices higher than the House of Representatives, at or below which sometimes some electoral constituencies are small and demographically coherent enough to allow people who don't meet Master Class approval to get elected). -- Yoshie