WASHINGTON (MarketWatch) — What is Elizabeth Warren’s game?

The Massachusetts senator has drawn attention in recent weeks for the ferocity and frequency of her attacks — many of them ad hominem — against the presumptive Republican nominee, Donald Trump.

It has led many commentators to conclude she is “auditioning” to be Hillary Clinton’s running mate if the Democratic front-runner wins the nomination in July.

But Warren, whose popularity with progressives lies in her single-minded opposition to the predatory practices of banks and other economic injustices, is in the process of squandering her hard-won credibility by taking the low road against Trump.

By so clearly aligning herself with Clinton and the presidential hopeful’s “pivot” to a general election campaign against Trump, Warren is losing whatever value she may have had in unifying the Democratic Party, let alone in becoming the vice presidential nominee.

Her new outburst is winning rave reviews from leftist commentators, such as Salon’s Gary Legum, who labels Warren the “Dems’ lethal weapon,” but their very praise shows the risk attached to this strategy.

“Watch the speech,” Legum says, referring to a rabble-rousing Warren peroration this week calling Trump a “small, insecure money-grubber” among other things, “and you can nearly see the contempt dripping from her lips and splattering on the lectern.”

And this picture of a politician whose stridency already puts off a lot of Democrats is one painted by a friend.

In fact, however, Warren has been puzzling and disappointing what used to be known as the Warren wing of the Democratic Party for months.

The disappointment started with Warren’s own refusal to run for president, despite her ability to whip up enthusiasm among Democrats for an authentically progressive agenda.

Her early failure to endorse Sanders, whose platform and passion mirror her own, might have been excusable. But her failure to endorse him before the Massachusetts primary in March and perhaps tip the scale in his favor in this pivotal vote started to look like a betrayal.

When Warren then failed to give Sanders some cover in demanding Clinton release the transcripts of her Wall Street talks — even though Warren herself has repeatedly criticized Clinton for her ties to the banks — it became clear that she had cast her lot with the front-runner even if she did not formally endorse Clinton.

A collection of comments from Warren’s Facebook page after the Massachusetts primary accused her of everything from buckling under Capitol Hill pressure to revealing her own political ambition and acting like a coward when courage was called for. “Wuss” and “sellout” were two of the names bandied about by disillusioned followers.

And the thing is, Warren will never be Clinton’s VP pick, so she has blown her credibility and sacrificed her leverage for nothing.

Because, aside from all the other reasons not to pair two women approaching 70 at the top of your ticket if your weak spot is white male voters, the Democrats will not risk regaining a majority in the Senate by vacating the Massachusetts seat and letting Republican Gov. Charlie Baker appoint a replacement.

The Democratic bench is so thin at this point that the same logic will disqualify other potential running mates, such as Ohio Sen. Sherrod Brown, who has the additional disadvantage of being completely unknown.

Trump, in fact, may have the easier task picking a running mate, especially since he has wisely decided to focus on veteran politicians.

If the recent buzz is right and he chooses someone like Tennessee Sen. Bob Corker, chairman of the Foreign Relations Committee, as running mate, he could at a stroke make his ticket look much more acceptable to a broader base of moderate Republicans, independents and working-class Democrats.

A survey by The Wall Street Journal this week of potential VP picks in Congress includes Warren as a possible choice for Clinton’s running mate, but notes concerns about two women on the ticket and even the possibility that Warren could “outshine” Clinton.

On the Democratic side, the article notes the Senate majority issue for Warren, Brown and to a certain extent for Cory Booker of New Jersey.

This leaves the colorless party apparatchik Sen. Tim Kaine of Virginia — no risk of him outshining anyone – or a congressman like Xavier Becerra of California, as well as the perennial Julian Castro, the former San Antonio mayor elevated to housing secretary to give him some Washington seasoning.

Trump’s potential choices include some of his former rivals for the nomination as well as Corker, Sen. John Thune of South Dakota or Sen. Jeff Sessions of Alabama.

Running mate or not, Warren, who has pledged to do everything to stop Trump from winning the White House, is likely to continue her terrier-like attacks. If Clinton wins and the Senate majority is not an issue, perhaps Warren will get a mid-level cabinet job in reward for her loyalty.

There are some who think Warren is playing a clever “long game” and will stay in the Senate with enhanced influence over an incoming Democratic majority.

If Warren and Sanders both remain in the Senate, however, she may well have lost her claim to leadership of the progressive movement to someone who actually stepped up to fight for the causes Warren has long talked about.