Trump sent his first campaign fundraising email last week.

Donald Trump needs money.

Yes, he is a billionaire many times over who lives in gold-plated luxury.

But if he wants to be president, he needs money from willing donors.

In the wake of reports that showed him desperately behind presumptive Democratic nominee Hillary Clinton by massive margins in raising campaign funds and him having just $1.3 million in cash on hand, Trump last week sent his first campaign fundraising email. In it, he promised to personally match the first $2 million raised.

He followed it up last week with another blast email, asking people to donate varying amounts to "indict" Clinton and promising to raise an additional $10 million.

He reportedly made his first goal quickly, with his campaign saying it was met in 12 hours and that he ultimately raised more than $10 million.

If so, it was a needed infusion of cash. While several reports indicated Trump's campaign was ill-funded and that it might spell trouble for his candidacy, few of them said why so little money is problematic.

For the answer, we turn to the example of Wisconsin Gov. Scott Walker, who was the first major candidate to drop out of the Republican primary despite a huge amount of support in the form of super PACs and the famous industrialist brothers Charles and David Koch.

The reason Walker dropped out -- his campaign was lagging in raising money directly.

See, super PACs are great for spending money on advertising kicking around an opponent.

But they are forbidden from spending money on a campaign directly or coordinating with a campaign on how to spend money.

They can't pay for things a campaign needs, including staff, travel, office space, paper, pens, computers and staplers. They can't even pay for late-night pizza for campaign staffers and volunteers to munch on.

This is what did Walker's campaign in. He had a lot of outside groups backing him, but because of sagging direct fundraising, couldn't afford to run his campaign anymore.

Now, $1.3 million -- or north of $10 million now -- is a lot of money. But for a nationwide presidential campaign, which can blast through millions in a day, it's a minuscule amount.

Trump has loaned his campaign gobs of money, but a presidential campaign is expensive, even for a billionaire.

Consider this, President Barack Obama and his opponent in 2012, Republican Mitt Romney, spent more than $1 billion combined. That figure doesn't count the nearly $2 billion spent by outside groups.

So The Donald, already lacking the extensive, experienced ground operation enjoyed by Clinton, will have to continue being a billionaire begging for money if he wants his campaign to survive.