When Sen. Amy Klobuchar broke through in the New Hampshire primary, relegating Sen. Elizabeth Warren to single digits and fourth place, the Massachusetts Democrat could not have been more gracious toward her party rival from Minnesota.

“I . . . want to congratulate my friend and colleague, Amy Klobuchar,” Warren declared, “for showing just how wrong the pundits can be when they count a woman out.”

It was part of an eloquent call for party unity in which Warren warned Democrats not to engage in “a long bitter rehash of the same old divides in our party,” and spoke with concern about a willingness “to burn down the rest of the party in order to be the last man standing.”

A speech that Democrats needed to hear got almost no coverage. So given what gets play (and Warren’s long-standing skepticism of financial institutions), it’s unsurprising that Warren directed some sharp criticism toward former New York mayor Mike Bloomberg on Thursday. Warren ­excoriated Bloomberg for once saying that anti-redlining laws prohibiting discrimination against minority neighborhoods in lending had helped cause the great financial crash of 2008.

“Anyone who thinks that,” she said, “should not be the leader of our party.”

So much for unity, you might say. Nonetheless, the party should not forget Warren’s earlier insistence that in the face of President Trump’s abuses of power, “we cannot afford to fall into factions, we can’t afford to squander our collective power.”

But it is Warren’s call to sisterhood that deserves more notice, partly as it relates to another underdiscussed divide in the party. One of the most striking findings of a New Hampshire exit poll suggested that female candidates actually do face electability concerns from voters that male candidates do not.

The Edison Media Research poll asked voters this: “If the Democratic nominee is a woman, do you think that it would make it easier to beat Trump, harder to beat Trump” or make “No difference.”

The poll found that only 9 percent thought that being a woman would make it easier to beat Trump. Nearly four times as many — 34 percent — thought being a woman would make it harder, and 55 percent said it would make no difference.

This gender issue appears to have affected outcomes in New Hampshire, particularly in the battle for second place between former South Bend, Ind., mayor Pete Buttigieg, who won the slot, and Klobuchar, who ran third.

The small group that saw a benefit to nominating a woman overwhelmingly favored Klobuchar over Buttigieg, 37 percent to 17 percent, and she also ran ahead of Sen. Bernie Sanders (I-Vt.) in this group. It gave the primary victor just 22 percent. Warren trailed badly with only 6 percent. To the extent there was a pro-woman vote, it appears that Klobuchar, not Warren, was its ­beneficiary.

But in the much larger share that saw a woman as having electoral liabilities, Buttigieg bested Klobuchar 23 percent to 16 percent, with Sanders winning 30 percent.

The race was a virtual tie among those who said gender made no difference: 25 percent for Sanders and 23 percent each for Buttigieg and Klobuchar.

There are limits on using exit polls to measure causality — whether a given answer accounts for why people voted as they did, or whether voters offer an answer that conforms to the choice they made for other reasons.

But it’s hard to deny that gender mattered to the New Hampshire outcome, and it will be part of the larger challenge Democrats face this year in avoiding the incapacitating factionalism that Warren counseled against.

Face it, Democrats: You are the diverse party and Republicans are the homogeneous party. Democrats include moderates and the left; Republicans are almost uniformly conservative. Among their elected officials, Democrats are the party of racial and gender diversity; Republicans aren’t. In the House, 37.9 percent of Democratic members are women, and 36.6 percent are African American or Latino. The numbers for the GOP: 6.6 percent women, 3.6 percent black or Latino.

Diversity is a source of pride for Democrats. But that pride must be matched by patience among the party’s ideological factions and its many different social groups, and by an embrace of the equal dignity of all members. The most important philosophical battles and group conflicts will be fought out among Democrats because Republicans, by the very nature of who they are, stand detached from these ­struggles.

Fighting exclusion while building electorally necessary solidarity isn’t easy. But for Democrats, there is no other option.

Read more: