Kate Zernike really captures what happened.
The key snippet:
By his second year as a senator, Mr. Paul had shattered the illusions of a good number of his supporters.
During the government shutdown of 2013, he was caught on a live microphone advising Senator Mitch McConnell of Kentucky, then the leader of the Republican minority, that the party could win the messaging war if it highlighted its willingness to compromise.
Mr. Paul reversed his pledges on issues like cutting military spending and foreign aid.