Republicans play hardball. Unions support the Dems, so they demonized unions as corrupt and anti-capitalist, and took measures to kneecap unions. And even after they successfully did that, they turned around and attacked gov't unions.
Dem policy proposals and even cultural positions are more popular than those of the GOP. So the Repubs attack voter turnout, kicking folks off voting roles, making it hard to register and vote (voter ID laws and such), reducing polling stations in minority areas, taking away voting rights from ex-cons, etc. They do whatever they can to reduce voting as they know their positions aren't enough to win.
If Dems played hardball like the GOP, they'd target farm states and rural states and the South which vote GOP. Cut back on farm subsidies, describing farmers as leeches demanding gov't handouts, etc.
Religious groups support and organize turnout for the GOP. Treat them like the GOP did unions. Go after their tax exempt status, especially for the groups who get into politics. Publicize their hypocrisy, their scandals, the ways in which they support the rich and go against the teachings of that Nazareth guy. Accelerate the downfall of religion, a retrograde institution blocking progress.
The Left also needs to build up a more concentrated network of think tanks and such to support those on the Left. Etc.
I give the GOP credit for understanding how to wield power, how to win with minority views, how to attack and discredit their perceived enemies.