Citizens United. In 2001, corporations were suddenly allowed to donate an unlimited amount of money to political campaigns. Campaign donations were deemed a form of expression protected by the first ammendment. The entrenched far-right republicans were suddenly being outspent by even further right people from out of nowhere, unseating many of them. The game for both parties then became how much corporate donation can you attract, and for Republicans, they found the further right you go the more votes you get. So republican nominees could have standards or keep their job, not both. The ones that were the far-right were still professional legislators, genuinely believing conservativism is in society's best interest, and many of them tried to resist, like McCain and Romney. The new far-right ones are professional shills in it for personal gain. The old ones are greedy cowards who know better. The Republicans have slid right at lightspeed ever since Citizens United.