WASHINGTON — The first rule of running for president is, do not talk about running for president — at least not until the last election is over.

"If anyone talks 2020, they should be disqualified, because our future is on the line," former Virginia Gov. Terry McAuliffe, a potential Democratic presidential candidate himself, said on MSNBC a few days before last Tuesday's midterm elections.

But now, the 2020 omerta has finally lifted, setting off a rush of activity from presidential hopefuls as they try to get the jump on the competition by snapping up support from donors, party officials and freshly unemployed political operatives coming off midterm campaigns.

"I think this is going to hit people like a truck later this week," Robby Mook, who managed Hillary Clinton's 2016 campaign and has informally advised a number of 2020 hopefuls, said just ahead of Election Day. "Activists are going to be bewildered by how quickly they're going to be asked to make a choice."

With President Donald Trump already campaigning for re-election and a wide array of Democrats looking to challenge him, there may be little rest for the weary as the so-called invisible primary for 2020 gets underway in earnest.

Most of the action will be on the Democratic side, where as many as two dozen potential candidates have been quietly — and in some cases not-so-quietly — preparing all year, campaigning publicly for midterm candidates in key states while privately courting crucial movers and shakers in the party.

"We've had in South Carolina in the past two months, close to 20 people," said Jaime Harrison, the former chairman of the Democratic Party in the crucial early primary state and a current Democratic National Committee associate chair. "I don't even know if I can remember them all."

But South Carolina, along with other early-bird states like Iowa, New Hampshire and Nevada, is about to get even busier as ambitious governors, senators, congressmen, mayors and a billionaire or two look to find a corner in a crowded marketplace.

"At the end of the day with a big field, you have to figure out a way to differentiate yourself from other folks. And if you wait until it's the quote-end quote 'right time for you,' you might be on the outside looking in," said Harrison.

Trump, 2020

On the Republican side, the starting gun went off long ago.

Trump officially launched his 2020 campaign by filing paperwork with the Federal Election Commission on Inauguration Day in 2017, and he's already raised a huge war chest of more than $100 million. He's also hired a small army of aides and advisers, including campaign manager Brad Parscale.

The president never really left the campaign trail, spending much of his first two years in office, and especially the latter part of 2018, barnstorming the country at trademark "Make America Great Again" rallies that served the dual purpose of supporting GOP candidates in the midterm elections and reactivating his personal connection with his base voters.

"People in my coalition are very happy, and they want more," said Matt Schlapp, chairman of the American Conservative Union and a close Trump ally.

Polls have consistently shown that nearly nine in 10 Republicans approve of his job performance and the vast majority are satisfied with his record — tax cuts, a growing economy, more spending on defense and two new conservative Supreme Court justices are among the items on the president's re-election checklist — even if they don't all love his style.

Dissatisfaction within GOP ranks, particularly among educated suburban Republicans, could prompt a primary challenge, even if it's a long-shot bid.

Trump will have to pay special attention to the three states that were key to his Electoral College victory in 2016: Wisconsin, Michigan and Pennsylvania. In all three, Republicans lost Senate and gubernatorial races in the midterms.

Trump campaigned in all three states in 2018 and yet two years into his presidency, with the economy in strong shape, voters in those states remain open to Democrats.

That portends a real fight for the president to either win back some suburban women or squeeze even more votes out of the rural strongholds that provided him big margins in 2016.

Outgoing Ohio Gov. John Kasich, one of the Republicans who lost to Trump in the 2016 primaries, is often mentioned as a potential contender in 2020 — either as a Republican or as an independent — and he is due to visit the first primary state, New Hampshire, on Thursday.

"We're in the option-gathering business," said John Weaver, a veteran GOP strategist and longtime adviser to Kasich.

Retiring Sen. Jeff Flake, R-Ariz., a frequent Trump critic, also recently visited New Hampshire. And Sen. Ben Sasse of Nebraska, another dissent Republican, has taken steps that could be seen as waters-testing, including writing two books critical of Trump's brass-knuckle style of politics.

The path to defeating an elected president in a primary, though, is a relatively unworn one. The only time it's ever happened was in 1856, when James Buchanan beat President Franklin Pierce for the Democratic Party's nomination.

For Dems, the question is when, not if

Democrats, meanwhile, face a free-for-all for their nomination, with no prohibitive front-runners and an epidemic of "why not me-ism?" breaking out among the party's rising stars.

Only one candidate has formally declared — little-known Rep. John Delaney of Maryland — but virtually all of the other potential contenders who have been mentioned in the news media have taken clear signs to prepare for a run.

And while no votes will be cast or caucus-goers tallied until Iowa kicks things off in February 2020, jockeying for position in 2019 will be just as important, since the party's delegate apportionment rules will likely winnow the field fast.

Former Vice President Joe Biden may cast the biggest shadow, but while he leads in what few 2020 polls exist, he's unlikely to clear the field the way Hillary Clinton did in 2016.

He probably can't even count on support from all of his Obama administration colleagues, some of whom have expressed interest, like former Massachusetts Gov. Deval Patrick, former Attorney General Eric Holder and Rep. Beto O'Rourke of Texas, among others.

Biden recently added three stops to promote his 2017 book, "Promise Me, Dad," about the death of his son, including some in the backyards of other potential 2020 candidates, including Sen. Bernie Sanders, I-Vt.

Sanders, the Democratic runner-up last time, may set the pace for the party's progressive wing and came off a recent campaign swing feeling energized by the large crowds that greeted him across the country, including in South Carolina and Iowa, according to people close to him.

But it's unclear if Sanders can get the band back together and reignite the spark of his 2016 insurgency without a foil like Hillary Clinton and with a number of fresher (and younger) faces for progressives to choose from — including several of his colleagues, like Sens. Elizabeth Warren of Massachusetts, Kamala Harris of California and Cory Booker of New Jersey.

For these highest-profile potential candidates, whose every move is scrutinized and who would enter the race with high expectations, timing can be tricky, Mook said, drawing on his own experience lining Clinton up for her April 2015 entry into the race.

"The longer you're in, the longer in you can raise money, the longer you can build your social media assets," said Mook. "But it’s also the longer Donald Trump can taunt you and the longer you'll be vetted (by the press)."

But for the rest of the potential field, who will hope to catch fire in the cornfields of Iowa and backwoods of New Hampshire, money may be the more immediate concern.

"For all these other candidates, the first question is: Where are you going to get the money?" said David Brock, a prodigious fundraiser who speaks with donors often and runs a constellation of Democratic groups and super PACs. "If you can't answer the question of where you're going to get the money, you're not going to go anywhere."

It's an ugly but real fact of American democracy: Running for president is expensive, and many a contender has sputtered out prematurely when they couldn't pay their staff or fly themselves around.

So it helps to be have connections to major donors, like McAuliffe, or be a proven online fundraising dynamo, like O'Rourke. And some candidates may bring their own money, which helps explain why so many billionaires — including Democratic mega-donor Tom Steyer and former New York City Mayor Michael Bloomberg — have had their names floated and been taken seriously.

"Part of it is self-fulfilling, because the more money you can get the more you can project an aura of viability, and money gets you more money," Brock said.