There is a meme that the Republican Party has a top-down, authoritarian structure where potential candidates patiently wait their turn for nominations. On the other hand, the Democratic Party hosts a divided, disorganized coalition. Yet this year, it’s the GOP that is seeing a roiling, rebellious bottom-up movement with little deference to leadership.

We have seen not one but two prominent Republicans, Sen. Arlen Specter and Gov. Charlie Crist, forced to leave the party because of insurgent primary challengers. Crist is a relatively popular governor who has not strayed far off the reservation, and polls well against a likely Democratic opponent. It is almost impossible to imagine a similarly popular Democrat getting so completely overwhelmed by a more liberal opponent that he or she would bail from the party before the primary.

On Saturday, long-term Senator Bob Bennett (R-UT) got booted at his party’s convention, losing the possibility of running in the primary for the nomination. The grassroots kicked him out of his own party, even though he is one of the most conservative members of the Senate.

Next Tuesday is the Kentucky primary. Polls indicate that Rand Paul will likely beat the establishment choice, Trey Grayson. Grayson was endorsed by Senate Minority Leader Mitch McConnell, probably the most powerful Republican in both D.C. and his home state of Kentucky. Grayson has the backing of the state and national party but is struggling to win the nomination. Even the recent Republican presidential nominee, John McCain, is seriously worried about losing to an insurgent candidate.

We are seeing a major shakeup on the Republican side caused, in part, by the evolving power of their grassroots members. The GOP is not a top-down party when the actual top of the party, Sen. McConnell, can’t secure the nomination for a classic conservative on his home turf.

The Democrats now emerge as the party of strong bosses. The biggest recent primary win against an establishment candidate was Ned Lamont’s defeat of Joe Lieberman. Even then, the Democratic establishment protected Lieberman, in opposition to the party base.

Similar challenges in the 2010 cycle are few. The only up-and-coming Democrats currently showing momentum in the polls: Joe Sestak, who might beat Sen. Arlen Specter in the Pennsylvania primary, in defiance of the party establishment, but that is only because Specter is a lifelong Republican who switched parties to save his seat. And Bill Halter, who has a shot at taking out Arkansas Sen. Blanche Lincoln. Specter is exactly the type of fake Democrat that primaries are supposed to keep from getting the nomination; Lincoln is not only an extremely conservative Democrat, but has little chance of winning the general election.

On the left, insurgent candidates struggle to beat even the most egregiously unprogressive establishment candidates. On the right, establishment candidates have left the party in fear of just the possibility of a primary challenger. Even those who are slightly too moderate for the local party are being taken down.