Immigration is not the only area where McCain's position has gone from independence to the Republican party line. He supported a "cap and trade" system for dealing with climate change as recently as 2008, when he told ABC's George Stephanopoulos that it would stimulate a green economy.
He coauthored multiple cap and trade bills in the past. But during this Congress he rejected Democrats' proposals for a cap and trade bill, switching to the Republican talking points that those bills would be too much of a tax burden – "cap and tax," he called them.
McCain was one of two Republicans to vote against the first iteration of President Bush's tax cuts. By 2006 he was voting to sustain them. And now, he thinks they should be extended. Democrats seem prepared to let the tax cuts expire.
McCain says he has not flip-flopped.
"I have not changed in my positions. I know how popular it is for the Eastern press to paint me as having changed positions. That's not true. I know they're going to continue to say it. It's fundamentally false," he told a reporter this week.
Charlie Crist's transformation in Florida went in the other direction, but has been no less remarkable. Faced with a challenge by former Florida House Speaker Marco Rubio, who excited conservative and Tea Party Republicans, he left the party altogether.
Rubio pilloried Crist for embracing President Obama's economic policies, particularly the $836 billion stimulus bill, early on, and it quickly became clear that if Crist wanted to see his name on the ballot in November, it would not be as a Republican. He left the party in April.
Since then he has moved further to the middle; his new constituency is independents and Democrats. He vetoed a bill that would have tied teacher pay to performance, endearing himself to teachers' unions.
While Crist still describes himself as "pro-life," He vetoed a bill that would have forced women seeking abortions to obtain an ultrasound. In previous years he had endorsed much stricter abortion language.
He now supports repealing the "Don't Ask, Don't Tell" policy on gays in the military
and softened his stance on other gay rights issues, like adoption.
Crist has also changed his tune on offshore oil drilling and has proposed a state constitutional ban, although the Gulf oils spill also played a role in that switch.
Crist has faced additional challenges since he left the Republican party. He now lacks the important structure for fundraising and getting out the vote that parties provide. But he is also unyolked at a time when political parties are not held in high esteem by many Americans.
"One of the more difficult things to do in politics is to win a primary contest as a moderate. On the whole, primary voters – Republican and Democrat – are wary of any candidate who strays from the party's core ideological moorings," said ABC News' Political Director Amy Walter.