It all started Wednesday night, as President Barack Obama gave his well-received healthcare address to a joint session of Congress. Breaching decorum, South Carolina backbencher Rep. Joe Wilson (R) twice screamed, “You lie!” when Obama promised that undocumented immigrants would not benefit from his reform plan.

{mosads}Wilson was wrong on the substance of his accusation, and his public disrespect of the president generated a storm of progressive outrage that quickly manifested itself in a fundraising bonanza for Democratic challenger and Iraq War veteran Rob Miller, back for a second effort after a shockingly close 2008 contest. In all, the fundraising blitz on Twitter and blogs like Daily Kos raised over $915,000 through the ActBlue clearinghouse, while Miller raised several hundred thousand more directly. By the end of the weekend, Wilson’s outrage — whether manufactured or genuine — had cost him over half a million dollars per word.

Just as importantly, the intense media attention filtered down into Wilson’s district . A poll by Public Policy Polling found that Miller had pulled into a statistical tie with Wilson, leading 44-43 in a head-to-head matchup. Last year, Wilson defeated Miller 54-46, and the district’s sizable African-American community, making up nearly a quarter of the population, may well be particularly motivated to turn out against a guy whose outburst is being categorized as racially tinged. Wilson did, indeed, vote with just six other state legislators less than 10 years ago to keep the Confederate flag flying over the state capitol.

At the end of the June 30 quarter, Wilson had $211,604 cash on hand, while Miller had $48,974. Now Miller is sitting on a war chest of over $1 million, all of it from small-dollar individual donations, and still rising.

Democrats who stand in the way of healthcare reform should pay heed. Nothing suggests that the netroots’ ability to raise this kind of money is limited only to candidates challenging Republicans. In fact, given the GOP’s extreme minority position in Congress, much activist attention is focused on those Democrats who are proving roadblocks to real reform by putting insurance companies first, rather than focusing on their actual constituents — the people.

When votes start getting cast, those who vote against a public option will generate the kind of ire that Wilson received, and activists will look for ways to punish those corporatist Democrats. Primary challengers are already being recruited in several Blue Dog districts, and votes against reform will generate additional impetus for more primaries. And while ordinary Americans will never be able to outraise the insurance industry and their millions of bribe dollars, they can generate enough money to wage credible campaigns.

Consequently, Blue Dogs should worry about being in the mainstream of their constituents. For example, Rep. Jim Cooper in the Tennessee 5th congressional district has been a longtime foe of real healthcare reform. Yet a Research 2000 poll for Daily Kos found that only 31 percent of respondents would vote to reelect him. Asked if they approved of his actions on healthcare, just 31 percent approved, including just 16 percent of Democrats. All told, 61 percent supported a public option, while just 28 percent opposed it.

Progressive activists are itching for primary challenges to reduce the influence of corporatist Democrats in Congress. Blue Dogs can either solidify their electoral standing by voting on behalf of their constituents, or they can invite primary challenges funded by the party’s well-connected and engaged progressive base.

Moulitsas is founder and publisher of Daily Kos (www.dailykos.com).