So now Alan Khazei is gone too, the latest and highest-profile Democrat to leap out of Elizabeth Warren’s way in the race to replace Republican Scott Brown in the US Senate.

Things were going pretty well for the social entrepreneur before Warren, a Harvard law professor and former Obama administration consumer advocate, got into this race in September.

Not extremely well, mind you. Though Khazei was the best-known and best-financed of the Democratic hopefuls, he was still lumped in with the others, dismissed by national Democrats transparently desperate to find a bigger name to take on Brown.

This must have smarted, because Khazei has done some things: He cofounded City Year, which has become vital to urban neighborhoods across the country and a model for a domestic peace corps program he helped create. The last time he ran, in the 2010 special Senate election, he picked up some serious endorsements, including the Globe’s. He has pushed vital issues on the trail, speaking about the plight of the poor and the middle class.

He has also shone a light on the corruption that is our system of campaign finance, refusing to take PAC money, decrying the influence of corporations, criticizing Democrats and Republicans alike for playing the game.

Even as he prepared to quit the race in our interview yesterday, he railed against the Supreme Court’s Citizens United decision.

“It’s an arms race,’’ he said. “Everybody has given up and said, ‘This is the way it’s going to be.’ The voice of the people is crowded out.’’

That sort of criticism and his call for Warren to reject PAC money made him unpopular with some Democrats in Washington.

But it was Warren’s candidacy that ended Khazei’s Senate ambitions, sucking the air out of his, and everybody else’s, campaigns.

“I’m an idealist, but I’m a pragmatic idealist,’’ Khazei said with a chuckle yesterday. He managed to remain cheery despite having to tell his campaign workers it was all over.

So, here we are. No disrespect to the four other Democrats still contesting the primary, but it appears Warren will cruise to the nomination. Unless something amazing happens, which, given 2010, one shouldn’t rule out.

But if I were in Warren’s camp, I wouldn’t be celebrating Khazei’s departure, because Warren needs a decent primary.

Sure, she’s incredibly smart, eloquent about the problems that plague working families, withering about the financial institutions that pushed them aside for profits, and relentless in her focus on how government allowed the whole mess to happen.

She seems like the perfect candidate to take on Brown, and polls show her running about even with the very popular incumbent.

But Warren has never run for office before. She is capable of rookie mistakes - witness this week’s flap over her politically silly comment, reported in The Daily Beast, that she “created much of the intellectual foundation’’ for the Occupy Wall Street movement.

It’s much better to make those kinds of mistakes in a primary than in a general election. The line between straight-shooting and shooting yourself in the foot is a fine one. A primary in which Warren would be forced to argue for her candidacy and defend her positions would help her to find her bearings.

It would also force her to make her case, even more passionately, across the state, starting now.

Really, we’ve barely seen anything of Warren yet. It’s not clear whether what makes her impressive now will hold up.

With Khazei gone, it looks like we won’t find out until she faces Brown. That’s why, even if you’re a Warren fan, Khazei’s exit is bad news.