As Kennedy’s career revealed, an effective senator needs both qualities — not just giving voice to important causes, but also following through with legislation and building alliances across party lines to change policy.

After three years in office, Brown can point to a few high-profile instances when he’s bucked his party. But his longer-term priorities — the issues on which he would stake his career — aren’t easily discernible. In the Senate, he’s held back on divisive matters like repealing “don’t ask, don’t tell,” often making up his mind after most of his colleagues have already weighed in. By then, it’s too late for him to have a major impact. Meanwhile, vital Massachusetts needs like medical research and renewable energy aren’t properly addressed. As a political moderate, Brown has major clout in a polarized Senate — but Massachusetts has too little to show for it. The problem is less with Brown’s political skills, which are obvious, or his centrist values, than in his conception of the job. He often seems to view being a senator as an exercise in political positioning.

Warren sells herself short with her big applause line: “I don’t care about Scott Brown’s truck; I care how he votes.” Yes, her vote would be more reliably in line with Massachusetts’ traditional liberalism. But the real promise in her candidacy lies deeper in her character. She’s a relentless striver whose life story represents the best of American upward mobility. As a young mother, she worked her way through community colleges and state universities to become the nation’s top expert on financial consumer protection.

And after earning an enviable job at Harvard Law School, she pushed her way into the political arena, wrangling with such renowned inside players as Larry Summers and Tim Geithner to achieve her goal of creating a consumer protection bureau. Her crowning achievement, the bureau guards the interests of average citizens contending with credit-card companies, student-loan holders, auto lenders, credit bureaus, and more. Anyone who’s felt powerless to escape a fee that seems unfairly imposed, or to cover an interest rate they didn’t bargain for, owes Warren a debt of gratitude.

The fact that she didn’t stay on the sidelines, enjoying her teaching gig, attests to her commitment to what she believes in. Those who say that Warren’s passionate advocacy won’t play as well in the clubby Senate should consider how much of a senator’s power derives from simply being sharper on the issues, from agreeing to take on the gritty work of writing legislation, and from having a plan in place to seize the political moment. And she’s not, in fact, the diehard Democratic partisan that Brown paints her as; she was a registered Republican for 10 years. She’s every inch her own woman.

On the campaign trail, Warren warns incessantly of a coming crisis in student-loan debt and the challenge it would present both for students and universities. This is a matter of no small concern in the higher-ed capital of the United States, and it’s easy to envision Warren becoming the Senate’s leading policymaker on higher education.

She also speaks passionately about medical research, a once-bipartisan priority that’s diminished under the Republican party’s ideological clampdown on government spending. Two years ago, for the first time in memory, the National Institutes of Health budget was reduced by 1 percent — this in a federal account that Kennedy twice succeeded in doubling over five-year periods. One of every five of those dollars goes to Massachusetts. Kennedy, for one, would never have let such a reduction happen. Brown voted in favor of the cut.

Federal research dollars are vital to workers across Massachusetts. The state’s leading institutions, from the Massachusetts Institute of Technology to Massachusetts General Hospital, depend on them. And much of the state’s economy grows out of those types of institutions; they seed the start-up companies that employ technicians, lab workers, accountants, and secretaries. Those jobs provide customers for copy shops, lunch places, and pharmacies. Medical research should be sacrosanct to any senator from Massachusetts. And it’s Warren, not Brown, who talks about it with appropriate urgency on the campaign trail.

For his part, Brown prefers to define himself as a retail politician, helping constituents directly with their problems. And, indeed, he’s gone quickly to bat for local constituencies like fishermen (who also enjoy the support of John Kerry and members of the House delegation) and even the Cambridge Innovation Center, for which he helped engineer a change in the law to allow more “crowd-funding” on the Internet, to incubate start-up businesses.

Those worthwhile moves don’t have the tectonic effect of increases in medical-research funds, but Brown deserves credit for them. Likewise, his support for the Dodd-Frank reforms was important in defeating a Republican-led filibuster. But Brown’s willingness to bridge differences between the parties would be more meaningful if he had a clear agenda. Then, he would provide cover for other Republicans to join him in bucking their leaders; he could actually shape legislation rather than wait and pass judgment on other senators’ work. But on the biggest issue of the moment, the looming “fiscal cliff” of automatic budget cuts and expiration of multiple tax cuts, Brown foolishly tied his hands by signing Grover Norquist’s anti-tax pledge.

The “fiscal cliff” is a situation that cries out for moderate leadership. A centrist Republican like former Senator Warren Rudman of New Hampshire would call out his own party for resisting any revenue increases through taxation, even when accompanied by record budget cuts. But Brown seems to lack the confidence or commitment to play that role.

Brown has also sacrificed some of his good will with Massachusetts voters by making personal attacks on Warren. After having milked every conceivable benefit out of the news that she identified herself as Native American at points in her teaching career, Brown returned to the theme as a closing argument. He thinks it helps him to portray her, without clear evidence, as an unwarranted beneficiary of affirmative action; it may make him seem like the more relatable figure. But relatable doesn’t cut it if you don’t excel at your job; and by campaigning on his personality, rather than his abilities, Brown seems to be bucking for his own form of affirmative action.

Warren would do more for Massachusetts.