How 6 Presidential Candidates Went From Friends to Foes

— -- What happens when friends run against friends for the presidency?

This year’s race for the White House may be a good indicator. Erstwhile political pals are now scuffling on the debate stage and at each other’s throats on the campaign trail.

Just take Democratic frontrunner, Hillary Clinton, who attended Republican frontrunner Donald Trump’s wedding in 2005. Today, the geniality has worn off as the two frontrunners go after each other. But they aren’t the only ones.

Here’s a look at how six presidential candidates went from friends to foes:

Donald Trump and Hillary Clinton

Then: The Republican frontrunner once lauded the former Secretary of State, saying she would make a “great president.”

“I know Hillary and I think she’d make a great president or vice-president” Trump wrote in a 2008 blog post.

Now: As both presidential candidates seek the role of commander in chief, there has been no shortage of attacks. While campaigning in New Hampshire, Trump joked with a rallier who compared Clinton to a barking dog in the audience.

“What was that -- a dog?” Trump asked when the dog barked.

“Hillary!” a man in the crowd shouted earlier this month. “Huh? Oh, It’s Hillary --oh,” Trump quipped.

Donald Trump and Ted Cruz

Then: The Texas senator praised Trump early in the election, constantly avoiding attacking or even questioning the Donald.

“I like Donald Trump, He’s bold, he’s brash. And I get that -- that -- that it seems the favorite sport of the Washington media is to encourage some Republicans to attack other Republicans,” Cruz said in July on NBC’s “Meet the Press.”

He added, “I ain’t gonna do it.”

Cruz echoed his refusal to engage in a “cage match,” during the third Republican debate.

Now: Bromance no more -- Cruz took the gloves completely off in the first debate of 2016. When the presidential hopeful was asked to comment on questions raised by Trump on his eligibility to run for the White house because of his Canadian birth, Cruz argued, “Interestingly enough, Donald J. Trump would be disqualified because, Donald’s mother was born in Scotland. She was naturalized.”

Trump interjected, “But I was born here.”

But it didn’t end there, Trump shot back, “There is a big question mark on your head. And you can’t do that to the party. You have to have certainty.” Cruz maintains he is eligible for the presidency.

Jeb Bush and Marco Rubio

Then: Bush and Rubio first met in 1998, at a time Bush was running for governor of the Sunshine State and Rubio was campaigning for West Miami city commissioner. Fast forward 17 years later, both men are running for president.

Just before announcing his candidacy in April, Rubio told ABC News’ George Stephanopoulos that he and Bush would “continue to be friends.”

“Jeb is my friend. He’s still my friend...We’ll still continue to be friends. I have tremendous admiration for him,” Rubio said. “I think it’s important to understand I’m not running against Jeb Bush. And I hope he’s not running against me. We are competing for the same job.”

In an interview with ABC News’ David Muir, Bush opened up also calling Rubio a “good friend.”

Now: Today, the long-brewing battle continues to boil. Rubio unveiled an ad recently calling Bush “desperate,” in response to the pro-Bush Super PAC, Right to Rise USA, decision to spend millions on attack ads targeting him.

While campaigning in Manchester, New Hampshire, Rubio repeated he’s had millions spent on him in negative attack ads.

“I’ve had over $20 million spent on attacking me --that’s not grassroots money. That’s money from the establishment,” Rubio said Thursday referring to the $22 million spent on him in attack ads -- $20 million of that from Right to Rise, Bush’s PAC.

Hillary Clinton and Bernie Sanders

Then: Sanders was in the House of Representatives when the Democratic frontrunner was leading former President Bill Clinton’s Task Force on National Health Care Reform.

Last week, Sanders took to Twitter to share a photo of himself and Clinton in more amicable times.

“To Bernie Sanders with thanks for your commitment to real health care access for all Americans,” Clinton appears to have written on the photo.

Now: As the race for the White House heats up, the pressure is on with Sanders giving Clinton a run for her money in Iowa and New Hampshire.

The Vermont senator has been scrutinized on many issues by Clinton, most recently on proposing a health care plan that “will never make it in the real world,” in Clinton’s words.

“I’m not interested in ideas that sound good on paper but will never make it in the real world,” Clinton said recently at a campaign event in Indianola, Iowa.

But Sanders isn't the only one taking criticism. The Vermont senator has attacked his main opponent, Clinton, consistently for "taking money from the big banks."

“Well, the first difference is I don't take money from big banks. I don't get personal speaking fees from Goldman Sachs,” Sanders said at the fourth Democratic debate, when asked what the biggest difference was between him and Clinton's position on breaking up the big banks.