Republican presidential candidate Donald Trump's campaign raised just $3.1 million in May, while Democratic rival Hillary Clinton brought in $27 million. Here's a breakdown of the two campaigns' finances. (Jenny Starrs/The Washington Post)

Donald Trump's campaign raised $3.1 million in May and ended last month with $1.3 million in the bank, a remarkably poor showing that will only heighten Republican concerns about his ability to run a serious general election campaign against presumptive Democratic nominee Hillary Clinton.

Trump's paltry fundraising last month is totally inexplicable given that he effectively secured the Republican presidential nomination on May 3 when he crushed Sen. Ted Cruz (Tex.) in the Indiana primary. Cruz ended his campaign that night. Ohio Gov. John Kasich dropped out the next day.

It also runs directly counter to assertions made Monday by Trump's now-deposed campaign manager, Corey Lewandowski, who repeatedly insisted that "money is pouring in" to Trump's coffers. (It's possible Lewandowski was referring to Trump's June fundraising. But if Trump did so poorly in May why would he suddenly improve in June amid a series of self-inflicted campaign wounds that have made it impossible for him to find a message, much less stay on it?)

Trump had 27 days as the presumptive nominee to raise money in May. A quick bit of division means he averaged $114,000 raised in those 27 days. As the presumptive Republican nominee. Who is running against the Democrat that Republicans most love to hate. With polls that showed the general election race close.

Speaking of Clinton, her May was a little bit different than Trump's. For one, she was still fighting a serious primary challenge from Bernie Sanders. Despite that, she raked in $27 million and ended the month with $42 million on hand.

That's a stunning gap and one that virtually guarantees Clinton advantages in every aspect of the campaign — from TV ads to staff to get-out-the-vote efforts.

Trump won't have the Republican National Committee to fall back on either. The RNC collected $11 million in May — $3 million of which came from a joint fundraising account with Trump. The committee ended the month with $20 million in the bank but $7 million in debt. If Trump can't raise lots more money — both for himself and for the RNC — it's possible that coordinated campaigns aimed at helping GOP candidates up and down the ballot may not be fully funded.

That creeping reality will set off a new round of worries about Trump's candidacy and whether he might not only lose the White House but also cost Republicans their Senate majority and maybe even their House one, too. Especially since Trump has pledged not to self-fund his campaign in the general election, meaning that he needs to not only start raising vastly larger sums of money but also do it very, very soon.

If fundraising is, at its root, a test of whether you can get people to vote for you with their checkbooks, Trump failed in May. And he failed with every possible advantage working for him: momentum, decent-to-good polling and, at least for part of the month, a Republican Party that seemed willing to unify behind him.

Trump may not need to raise as much money as Clinton to win. But he can't have another month where he is outraised by her at a nine-to-one clip. If he does, the general election may be over before it ever really started.