Editor's note: David Gergen is a senior political analyst for CNN and has been an adviser to four presidents. A graduate of Harvard Law School, he is a professor of public service and director of the Center for Public Leadership at Harvard University's Kennedy School of Government. Follow him on Twitter at @david_gergen. The opinions expressed in this commentary are solely those of the author.
(CNN) -- Hillary Clinton's book roll-out the past few days has shown, as expected, that she still commands an army of rapturous fans, but the best news for her this week came from the completely unexpected: the defeat of Republican leader Eric Cantor.
Rarely has a GOP primary for a seat in the House of Representatives had such wide repercussions. For Clinton, they almost all run in her favor. Consider just three:
(1) Prior to the Cantor defeat, the GOP "establishment" thought they were starting to persuade middle-of-the-road voters that their party is not hostage to the tea party. After all, GOP establishment candidates so far this year have won five of seven Senate primaries where they were challenged by tea party candidates, most notably Mitch McConnell's victory in Kentucky. A public narrative was emerging that tea partiers remained a potent and, yes, welcome force in the GOP but that they aren't in charge.
The defeat of Cantor was a thunderclap transforming the landscape. Because Cantor is perceived as a strong conservative, and yet lost to an unknown on his right, the narrative has changed: Now the story line is that the GOP base is not only angry but demands even tougher conservatism and even less compromise.
All this plays directly into Clinton's hands. If she formally declares, she can now stand before the electorate and assert with greater credibility that she -- not the GOP -- represents sensibility and moderation, a leader eager to work across the aisle. She even has greater latitude to embrace some of Elizabeth Warren's positions, appealing to the rise of a populist left, so long as she doesn't veer too far from the middle (where most voters still are).
(2) Cantor's defeat all but extinguishes lingering hopes that Congress and President Obama can work out a bipartisan agreement on immigration reform. Two post-election polls have suggested that Cantor's perceived openness to immigration reform was not the decisive factor in his defeat, but his opponent certainly seized on immigration as a stick to beat him over the head as an out-of-touch, wrong-headed reform advocate.
In the wake of his defeat, it's hard to see how other Republican incumbents will follow Cantor's path. And for starters, they will likely be very wary of voting for immigration reform until dust settles.
That, of course, is bad news for the GOP presidential nominee in 2016 and good news for Clinton if she is the Democratic standard bearer. Mitt Romney could only garner 27% of the Hispanic vote in 2012, far below the 40% that pundits think is mandatory for victory. Unless it finds a way out of the cul de sac, the GOP share could sink even lower this time.
(3) We haven't yet heard from that corner, but one can only imagine that the Cantor defeat, tied so closely to charges that he was for "amnesty" for illegal immigrants, is prompting second thoughts by Jeb Bush and his many admirers about whether 2016 is the right year for him to run. If Bush's deep belief in immigration reform would tear apart his party in the primaries and make his path to the White House perilous at best, why should he?
If Jeb Bush decides not to go, that would be extremely welcome news to the Clinton camp. From the point of view of Democrats, the most obvious threat to a Clinton victory is a Republican governor conservative enough to unite his party but moderate and experienced enough to pull in purple states. Jeb Bush and Chris Christie are the two who come closest to that profile.
Conservatives may disagree -- many genuinely believe that a Rand Paul can win -- but Democrats will quietly celebrate if neither Bush nor Christie tops the Republican ticket.
It is an oddity of American democracy that a tiny number of voters -- fewer than 8,000 -- in a suburb of Richmond, Virginia, can substantially alter the national outlook. But we live in odd, unpredictable times. No doubt other big surprises lie ahead before November 2016.