With a huge push from the Tea Party at the height of its influence, and at the nadir of the economic crisis, Brown won a special election for Kennedy's seat in early 2010, robbing Senate Democrats of the 60th vote they needed to break serial GOP filibusters.
The consequences were immediate and chaotic. His victory turned a fragile Democratic consensus in favor of passing health care reform on its head, and nearly sank the initiative entirely. It forced House Democrats to swallow inferior legislation because Senate Democrats could no longer pass any sort of compromise legislation through regular order. Frustrated House Republicans were forced to settle for a modest list of refinements to the law, which Senate Democrats could pass via the filibuster proof budget process, and eroded trust between top Democratic leaders of both parties.
Recognizing his own vulnerability, Brown did partner with Democrats on key issues like Wall Street reform, but positioned himself to extract significant concessions from bill writers seeking more thoroughgoing rules for the financial sector.
At the time, Warren was an outside advocate for those rules. She was the originator of the idea that Congress should create a federal agency to protect consumers from malicious financial actors. And when the Dodd-Frank Wall Street reform bill passed, the Obama administration appointed her, in an interim capacity, to stand up that agency -- now known as the Consumer Financial Protection Bureau.
For months, Brown proved a resilient opponent, despite being a Republican in the most Democratic state in the country. He used the credibility he'd earned by voting with Democrats on signature issues to blunt Warren's argument that he did not truly represent the state's values.
But Warren prevailed in large part by taking the campaign national. She caricatured Brown, accurately, as a fair-weather bipartisan, willing to work with Democrats when his vote wasn't absolutely essential, but ready to stand with his own party when ever his vote was necessary to thwart liberal initiatives. And she argued effectively that re-electing Brown could toss control of the chamber to the Republican party, largely comprised of politicians who are far more conservative than Brown, who is relatively moderate and pro-choice.
Without this victory Democrats would be staring down a long night, hanging on to control of the Senate by a thread.
Brown was gracious in defeat and when his supporters booed Warren during his concession speech, he asked them to be gracious too. "She won it fair and square, folks."