Demise of American independents

The weaker US party system creates less incentive for independents than Australia, but since 1990 Vermont's sole representative in Congress has been independent and avowed democratic socialist Bernie Sanders. Vermont is the type of north-eastern state that was once moderate Republican but now with the reversal of the civil war polarity and the conversion of the Republicans into a southern conservative party it has become a Democrat stronghold. Sanders won election against Democrats and Republicans and for some his election has been seen as a model of progressive politics. Says one supporter:
this is not a man who is independent in the “between Democrats and Republicans” way - this is one of the most committed progressives ever to hold federal office, a man who is an independent because he has long believed the current political system is bought by Big Money. Electing him to the U.S. Senate transcends this election because it would elevate one of the strongest voices progressives to the national stage - a stage that, beyond a handful of courageous leaders like Ted Kennedy and Paul Wellstone, has been sorely lacking strong progressive voices for years.
Sanders is now running for the Senate. But is striking that his lower house seat he has no independent successor, indeed the likely Republican candidate Martha Rainville is rated a chance, if a low one, of winning the seat, her website doesn't seem to mention that she is a Republican. I cannot find anything on Sanders' website about the Representatives contest. Democrat leaders in Vermont are working to allow Sanders a clear run even though he has announced he would refuse to accept the Democratic nomination if offered. here is a clear example of how the American left is incapable of organising outside the Democratic party, in his local government career before 1990 Sanders had his own party-type organisation, the 'Progressive caucus' but this seems to have evaporated. . Sanders seems indistinguishable from the Progressive caucus in the Democrats, of which he is the only non-Democrat member and which he helped found. Perhaps George Bush has encouraged the demise of the non-Democrat left.