BOSTON — Elizabeth Warren is the kind of person Massachusetts has always liked to send to the U.S. Senate.

She would instantly become a national leader, which appeals in a state that has sent to Washington Democrats such as John and Edward Kennedy and Republicans such as Henry Cabot Lodge and Edward Brooke. The Harvard Law School professor who warned of abuses in the financial system long before the economic crisis should draw suburban liberals who admire her seriousness as well as lunch-bucket Democrats who appreciate her populism.

And few in her party have made a more compelling case that successful capitalism requires a dose of government to guarantee fair competition, economic justice and the public infrastructure businesses require. Warren’s off-the-cuff statement on the subject a year ago was so eloquent that it went instantly viral.

So why hasn’t one of this year’s most exciting Senate candidates put the election away? The obstacle is a Republican incumbent who is making voters forget that he’s a Republican. If former House Speaker Tip O’Neill preached that all politics is local, Sen. Scott Brown makes all politics personal. He’s running even or, in one recent poll, slightly ahead of Warren simply because so many voters like him.

The best summary of what’s happening is offered by Dick Flavin, a veteran of the Massachusetts political wars. “A lot of people vote for how they feel about a candidate, not what they think about a candidate,” Flavin said, “and she’s doing the think stuff.” Brown can walk into a tavern and make people feel he’s one of them. Thus his simple slogan: “He’s for us.”

Brown has also been good at picking issues where he can separate himself from his party — even as he often votes with the Senate Republican leadership. When Rep. Todd Akin made his outlandish comments about rape, Brown sensed not danger but opportunity. Speaking as “a husband and father of two young women,” he won banner headlines by quickly condemning Akin and urging him to drop out of the Missouri Senate race.

Brown won a celebrated special election in January 2010 as a tea party hero who traveled the state in his pickup truck and pledged to fight President Obama’s health care plan. Ever since, Brown has cast himself as a middle-of-the-roader who can work with everybody, including local Democratic officials. Todd Domke, a Republican consultant, said that even “moderate-to-liberal Democrats” here warm to “the idea of a moderate Republican who breaks with his party.” The state’s many independent voters relate to the profile Brown has painstakingly built.

But Brown is different from classic Republican moderates (such as Brooke) who battled conservatives inside the party. He more fits retiring Rep. Barney Frank’s quip that “moderate Republicans are the people who are there when you don’t need them.”