Warren just emphasized her biggest weakness

Sen. Elizabeth Warren, D-Mass., has arguably had the best summer of any presidential candidate. She had two solid debate performances, built an impressive ground game in early states, steadily chipped away at Vermont Sen. Bernie Sanders' support, and managed to climb into second place in a number of national and early-state polls.

Her steady stream of policy proposals has convinced voters and the media she is serious and prepared for office.

That said, she still has two interrelated weaknesses.

First, voters desperate to beat President Donald Trump don't know if she is too progressive (especially on Medicare-for-all) to win more centrist voters.

Second, and arguably more important, her misstep on her Native American ancestry raised serious concerns about her political acumen, suggesting Trump is going to have her on defense on Day One.

So as the summer draws to a close, what does she do? Put a neon light around her biggest misstep. The Washington Post reports: "Sen. Elizabeth Warren opened her remarks at a Native American presidential forum with a more straightforward version of the apology she has offered in the past for identifying as a Native American for two decades while she was a law professor."

She said contritely, "I want to say this, like anyone who's been honest with themselves, I know that I have made mistakes." She went on: "I am sorry for harm that I have caused. I have listened and I have learned a lot, and I am grateful for the many conversations that we've had together."

I've got no idea why she chose to make this a headline - again - just as the issue was dying down and she was managing to erase concerns about electability. Her nomination is not imperiled because she made a good-faith mistake in failing to understand tribal heritage. She is at risk because people think she's a naive candidate who plays into Trump's hands.

If you are another Democrat in the presidential primary race, you're probably doing a little celebrating. No one in the field has really laid a glove on her in the debates. She's a whip-smart candidate with a tremendous work ethic, so it's hard to corner her or outwork her. Democratic voters don't like infighting, so it's hard for competitors to get too aggressive. The one person who definitely can beat Elizabeth Warren is Elizabeth Warren.

It was bad enough creating the issue in the first place. There is no plausible excuse, however, for digging it up a second time. Is she too hung up on not giving offense? Does she have a political tin ear? Does she not understand the need to project confidence? Maybe it is a little of all three.

A progressive polling outfit, Data for Progress, just came back with survey results showing just how important electability is:

"Even the leftmost wing of the Democratic Party places the ability to beat Trump as the most important candidate characteristic, though they also place more weight on additional policy goals. Progressives and Socialists placed above average weight on climate action, progressive taxation, Medicare for All, and abortion rights. Moderates placed an above average weight on compromise with Republicans, connecting with rural America, and experience. However, even among Moderates, these characteristics were not rated as more important than the progressive policy goals that the left-wing of the party placed as top priorities."

In short, Democrats this year are largely one-issue voters: The nominee must beat Trump. To the extent Warren has committed a second unforced error on the very same issue, and put front and center the issue as to whether she can take on Trump, she will have succeeded only in re-raising the deepest worry Democrats harbor: picking a losing candidate.

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