Entryism. By definition it seems to be the left-wing moving more extreme rather than non left-wing all moving together towards the right. Democrats have shifted more to the left than Republicans to the right: https://archive.is/IILDt

Nah, what right wingers want has stayed remarkably similar over the decades, and many of their stances have in fact become more left-leaning over the years. What has changed is that they're more willing to fight for what they want, and not let the left unilaterally dictate what's respectable. That increased stridency is very real, but their stances and wants haven't become markedly more extreme over time, if anything the opposite.

There's multiple polls and investigations that show that it's the left that has adopted new and more extreme stances and has become less tolerant of dissent from those new goals. eg. Obama's border policy would be unthinkable in today's no human is illegal ideological climate.