Editor's note: John P. Avlon is a CNN contributor and senior political columnist for The Daily Beast. He is the author of "Wingnuts: How the Lunatic Fringe Is Hijacking America."
(CNN) -- Tuesday marks the final round of the primaries before November's general election. Seven states will be voting. But two contests in particular symbolize the struggle we've seen all year between the centrists and ideological activists.
One is the ugly GOP Senate race in Delaware between Mike Castle and Christine O'Donnell; the other is the Washington, D.C., Democratic primary for mayor, in which incumbent Adrian Fenty is trailing in the polls. Both are high-stakes local elections with national implications.
It's New Jersey Gov. Chris Christie vs. Sarah Palin and the Tea Party Express in Delaware's GOP Senate primary. The Christie-backed Castle is a classic Northeast centrist Republican -- an increasingly endangered species -- but his politics are a great fit for the state, which helps account for the fact that he is the state's longest-serving congressman, widely seen as honorable and effective. Polls indicate that only Castle can win Joe Biden's old Senate seat for the GOP. But longtime social conservative activist O'Donnell is trying to take Castle out in the primary -- with the support of Palin, South Carolina Sen. Jim DeMint, Rush Limbaugh and the Tea Party.
This race exposes the fault lines in the GOP -- Tea Party supporters have always pushed the notion that their movement is about a focus on fiscal conservatism with a strong libertarian streak. But as governor in the 1980s and 1990s, Castle compiled an impressive fiscal record as the protégé of Pete DuPont, cutting income taxes three times and balancing the budget.
In Congress, Castle voted against the stimulus bill and health care because he believed they were too damaging to the deficit. However, Castle voted for cap-and-trade climate change legislation, is pro-choice and forcefully called out birthers at an early town hall meeting.
Being called a RINO -- a Republican in name only -- is not the only slander he's being attacked with in this campaign: O'Donnell campaign aides have been pushing baseless rumors that the married and Roman Catholic Castle is gay. It's an ugly attempt at dividing to conquer, and comes with the added baggage that O'Donnell, as a founder of the 1990s evangelical anti-abortion and pro-abstinence group "Saviors Alliance for Lifting the Truth," advocated the idea that gays and lesbians could be "cured" and converted to heterosexuality.
She's never held elected office and only recently got her college degree -- essentially, she's been a professional social conservative activist. O'Donnell is hoping that a low-turnout primary might allow her to pull off an upset, but polls predict that she would lose the Senate race in the general election by double digits, while Castle would win against the Democratic nominee by a similar margin. So the question is not only the center vs. the far right, and Christie vs. Palin. It's electability vs. self-sabotage.
The Democratic-dominated Washington, D.C., primary is essentially the general election when it comes to selecting the mayor of our nation's capital. So when The Washington Post came out with a poll two weeks ago showing incumbent Mayor Fenty losing the September 14 primary to his chief challenger, City Council President Vincent Gray, despite residents' thinking that Fenty has done a good job for the city, it should have sent a shock wave through every centrist Democrat. There are several issues that affect Fenty's popularity, but a major one is his effort to reform the schools. The poll results show the measure of the teachers union's political strength in the Democratic Party. This could signal a major backlash against education reform nationally.
Fenty and his school chancellor, Michelle Rhee, have been close allies of the Obama administration in backing merit pay and charter schools, effective reforms that have infuriated "educrat" critics who like the status quo.
Education reform is one of the few cutting-edge policy areas in which Republicans and Democratic reformers increasingly agree. (The upcoming documentary "Waiting for Superman" shows the fight on the front lines.)
If union strength is sufficient to help take out an incumbent mayor in a primary, despite overall high approval ratings from residents, it will send a chilling message to other mayors across America who have been embracing education reform and finally seeing some student gains. It will also be interpreted as a brush-back pitch against the Obama administration from a key constituency.
Public sector unions are to the Democratic Party what the religious right is to the Republicans -- the key suffocating special interest that demands fealty and targets centrist reformers who try to exert independence.
Whatever the results of Tuesday's primaries, it's already clear that low-turnout, closed partisan primaries can be hijacked by ideological activists.
Both Castle and Fenty probably would be elected in general elections, when all voters can participate and turnout dramatically increases. The closed partisan system -- combined with the rigged system of redistricting -- is making the two parties more polarized and less representative, leading directly to the angry hyper-partisanship that is alienating a growing number of Americans.
These two races Tuesday are tests of whether centrist candidates can survive. For the moderate majority of Americans, it's time to give a damn and take a stand.
The opinions expressed in this commentary are solely those of John Avlon.