No one exemplifies the dynamic better than Mrs. Lincoln. Over her 12 years in the Senate, she’s been careful to project herself as a Democrat in tune with Arkansas voters and business. The party leadership’s decision to push card check and the public option (both highly unpopular with the general public and the Arkansas public) forced Mrs. Lincoln to push back, which cast her as the spoiler of liberal dreams.

Mr. Halter was the result, propelled from the start by groups such as MoveOn.org. The lieutenant governor has run far to Mrs. Lincoln’s left, and in March, his first month of campaigning, he raised more than $2 million. And the left is unleashing money against his opponent; the Service Employees International Union recently unveiled a $1 million ad campaign against Mrs. Lincoln.

Win or lose, the base’s candidates are pulling the Democratic field left. Colorado’s appointed Sen. Michael Bennet was intending to win re-election by keeping his head down, splitting the difference on tough issues. Then, last September, the grass roots fueled former Colorado House speaker Andrew Romanoff’s entrance into the race, who announced his support for an ObamaCare public option. Not to be outdone in a closed Democratic primary, Mr. Bennet became the Senate’s most vocal public-option supporter.