Democrats all want things from the government, so they are united and motivated to win.
Republicans aren’t usually angling to get programs and projects from the government, so they are less united and less motivated.
When Republican politicians are motivated, it may be because there’s something they want from the government.
Much of the time, though, they are content to be ineffectual placeholders.
Republicans are the party of “no” which is fine if you have alternatives which they never do.