I guess it is natural in a competitive politics that Republicans would trash the genuine achievements of a Democratic Administration. No wonder Donald Trump finds now how 'economic growth' and 'sustained employment gains' would not work for many common Americans. Do you detect similarity in his message and that of Bernie Sanders?
The question is why years over years, common lower middle class of America consistently votes Republicans who have followed policies that only help the 'rentier class'. For decades, policy of Republicans has been 'tax cuts for rich' even though there is no observed relationship between those tax cuts and income growth for common person. Why has lower middle class of America consistently been hood winked by the 'trickle down' economics?
Fear of National Security and Cultural War are obvious reasons for that. Whether Karl Rove machinations like putting 'same sex marriage' on state ballot to drive evangelicals voter block to Bush even after he had emptied American treasury by Trillions of dollars in giving hefty tax cuts to rich as well as hubris of invading Iraq or Ted Cruz's language complete with Biblical references or Carly Fiorina's over the top lying about Planned Parenthood or across the board opposition by Republicans to any sensible gun control regulations; the pattern is very clear - keep diverting attention of Americans from issues on hand or real solutions which will solve those problems.
Donald Trump's innovation in 2016 cycle is to add 'white ethino-nationalism' explicitly to this mix with a good dollop of 'anger'. That is a pretty powerful weapon Trump has exploited and it is not given that the so-called demographic edge of Democrats would blunt it. Until and unless Bernie Sanders, Hillary Clinton and Democrats expose this 'white ethino-nationalism' and refute the trashing of legitimate achievements of a Democratic Administration; Trump's winning ways may be unstoppable.