Eric Cantor
House Majority Leader Eric Cantor lost his primary in a stunning upset. U.S. House of Representatives

David Brat, an unknown Virginia college professor, rocked the Republican establishment Tuesday night by defeating House Majority Leader Eric Cantor in his primary. The shocking upset showed that reports of the death of the Tea Party had been greatly exaggerated, as Brat’s campaign received a boost from being backed by leaders of the conservative movement.

So why did Cantor lose? Here are five reasons:

1) The race was a referendum on immigration "amnesty"

And Cantor’s position turned off voters. The majority leader supported legalizing some children of illegal immigrants, which opponents call "amnesty," and Brat relentlessly attacked him for it. While Cantor tried to scrub his record of supporting compromise on the issue from his campaign website, Brat reminded Republican primary voters where Cantor stood.

"The central policy issue in this race has become Cantor’s absolute determination to pass an amnesty bill. Cantor is the No. 1 cheerleader in Congress for amnesty," Brat said, according to Breitbart, a conservative news site. "This is not the Republican way to fix our economy and labor markets."

2) The Tea Party is not dead

Although Brat wasn’t heavily funded by national Tea Party groups, the movement’s message struck a chord with Republican primary voters in Virginia’s conservative 7th Congressional District. Brat was also endorsed by Tea Party favorites Ann Coulter and Laura Ingraham, who appeared with him on the campaign trail.

3) Brat highlighted Cantor’s ties to big business

The message -- that being beholden to big business was to the detriment of Cantor’s constituents -- worked on Republican primary voters.

“If you’re in big business, Eric’s been very good to you, and he gets a lot of donations because of that, right?” Brat said at a Hanover County Republican Party meeting in April, according to Politico. “Very powerful. Very good at fundraising because he favors big business. But when you’re favoring artificially big business, someone’s paying the tab for that. Someone’s paying the price for that, and guess who that is? You.”

4) Negative campaigning

Cantor outspent Brat by 40-to-1, or $5 million to Brat’s $120,000.

“Much of that money went to negative ads against Brat that turned off voters and were so vitriolic as not to be credible,” said conservative columnist John Fund of the National Review.

Fund was referring to a Cantor ad that denounced Brat for "working for" then-Gov. Tim Kaine, a Democrat who is now Virginia's junior U.S. senator. In fact, Brat was appointed by Kaine to the Virginia Council of Economic Advisers – a post he was also reappointed to by Kaine’s Republican successor, Bob McDonnell.

5) He’s unpopular

A Public Policy Polling survey from Tuesday showed 49 percent of Republican voters in Cantor’s district disapproved of his job in Congress, 43 percent approved of his performance and 8 percent were unsure. Among Democrats, Cantor is even more unpopular, with 91 percent disapproving of him.