It's been a pretty good week so far for Hillary Clinton in that it has been a pretty terrible week for Donald Trump. He fired his campaign manager just hours before posting what is almost certainly the most pathetic monthly fund-raising total for a major party's presumptive presidential nominee in modern history, leaving him with less than $1.3 million in cash. As of the most recent figures available, 82 committees for House of Representative candidates reported more money than that. Mitt Romney raised about 27 times as much in May of 2012 as Mr. Trump did in May of 2016. Heck, the dregs of Mr. Romney's 2012 presidential campaign are still about as flush as Mr. Trump's with more than $1 million in the bank at the last count. Meanwhile, Mr. Trump's campaign has more than $45 million in debt, mainly to the candidate himself.

At this stage of the game, that might be more significant than any poll numbers, though in key battleground states, those figures are looking pretty good for Ms. Clinton, too. Now is the point in a presidential election when candidates are building state-by-state organizations to mobilize support and get out the vote in November. And particularly flush campaigns, like President Barack Obama's in 2012, can use the lull between the end of the primaries and the party conventions to begin to define the opponent in the worst possible way. He did that to devastating effect against Mr. Romney in 2012, and Ms. Clinton is doing the same against Mr. Trump now. She and allied groups are up with $23 million worth of ads in battleground states, to Mr. Trump's $0.

But for Ms. Clinton and the Democrats, now is no time to get cocky. Mr. Trump has thus far violated every known rule of modern campaigning, yet he stands as the Republican standard-bearer after defeating a slew of candidates who outraised, outspent and out-organized him throughout the primaries. Just as he was able to get away with saying things that would sink anyone else, he's gotten away with spending just a fraction of what his rivals do on television ads. The trend is so stark that local TV stations are starting to worry about their revenues for the fall. Yet the Trump campaign has rolled along on a sea of free media — not just through his prodigious use of social media but also through the tremendous coverage he has generated in print, television and the web.

Ms. Clinton is a trailblazer in the respect of being the first woman to head a major party's ticket, but in all other respects, she is a conventional candidate in a highly unconventional year. This is not to say that she doesn't have substantial advantages, just that she can't assume that they are the right ones for 2016.

That doesn't mean she should fashion an outrageous persona for herself on Twitter or shift a couple hundred grand into spending on hats. Rather, she should show a bit of boldness when it comes to the one thing she can do right now to break through the Trump domination of the news media, and that is her selection of a running mate.

Ms. Clinton is reportedly deep into the vetting stages of potential vice presidential candidates, with speculation centering on three possibilities: Sen. Tim Kaine of Virginia, Housing and Urban Development Secretary Julian Castro, and Sen. Elizabeth Warren of Massachusetts. All have compelling attributes. Mr. Kaine could provide cross-over appeal for independent voters in swing states. Mr. Castro is telegenic, young and Latino — a walking rebuke to Mr. Trump's overtly racist statements toward Hispanics. And Ms. Warren would not only double-down on the history-making of a female president but also appeals directly to Sen. Bernie Sanders' supporters.

But if the question is who would best bring the fight to Mr. Trump, the answer is clearly Ms. Warren. The ever-growing listofwebheadlinesinvolvingthewords“Warren,”“Trump”and“epic take-down,” or variations thereof, attest to her willingness to relish a running-mate's role as an attack dog, and in a year when voters want a break with politics as usual, she provides it. The more stories about Wall Streeters threatening to cut off funds for the Clinton campaign if Ms. Warren is on the ticket, the better, as far as that's concerned.

Granted, there's a history of some bad blood between Ms. Clinton and Ms. Warren, and if elected, Ms. Clinton will need a No. 2 she can implicitly trust. But even if the chemistry between the two is such that Ms. Warren isn't the right choice, Ms. Clinton needs to be looking for someone who brings to the race what the Massachusetts senator would. The list of candidates who thought Mr. Trump would be easy to beat is long. Ms. Clinton can't afford to become one of them.