Hillary Clinton has a healthy lead over Donald Trump in the polls. Republicans look likely to lose control of the Senate. Could Democrats hit the trifecta — and also take back the House of Representatives this fall?

It’s not impossible.

Even though most analysts think it’s unlikely — Democrats would need to pick up 30 seats, including in reliably Republican territory — there is one political analyst who has a theory of how it could happen.

Geoffrey Skelley, who closely tracks congressional races at the University of Virginia’s Center for Politics, suggests there’s a crude shorthand for evaluating the battle for the House: Look to see if Clinton can beat Trump by 6 points or more in the presidential race. If that happens, Skelley projects 50 seats would be in play.

If Clinton wins by 6 points — and current polling suggests she might — it would mean Democrats could have a real, genuine, honest-to-goodness shot at pulling off what looked impossible just a few months ago.

By most projections, Democrats look more likely to get closer to 15 or 20 seats, not the 30 they’d need. Many states have gerrymandered safe Republican seats that would require an extraordinary landslide to do the trick. In 2012, for instance, Democratic House candidates won 1.7 million more votes than their Republican foes — and still ended up with 33 fewer members of the House. This is why even many Democrats believe taking the House is unlikely.

But unlikely doesn’t mean impossible. Here’s the math behind how Democrats’ long-shot dream might just become a reality.

Projection: A 6-point Clinton victory would put 50 Republican-held House seats in play

Get a district to vote for your party’s presidential nominee and your party will also probably win its House seat.

That’s not an ironclad rule, but it’s a pretty good indicator — in 2012, only 6 percent of districts that voted for Barack Obama voted a Republican into the House.

This is the key to understanding why Skelley thinks a 6-point Clinton win could put the House in play. That kind of national victory would likely mean 50 House districts currently controlled by Republicans would vote for Clinton — therefore suggesting they have a good shot of also going blue at the House level.

Of course, a Clinton win in these 50 districts wouldn’t guarantee House Democrats will pick up all of those seats. (Many are held by powerful or longstanding Republican incumbents who are well-funded and enjoy good reputations at home.) But it does mean that Democrats could lose 40 percent of the House races in districts won by Clinton and still take back control of the House.

And here’s the interesting thing: The polling suggests this is not only a possibility but exactly what’s projected to happen. Averages of all the major polling firms compiled by both RealClearPolitics and the Huffington Post currently put Clinton’s lead right around the 6-point mark.

"A 6-point lead would help create the conditions for Clinton to pull the Democrats over the finish line to take the House," Skelley says. "It doesn’t mean they would, of course, but it begins looking really possible."

What happens if Clinton wins by 4 or 5 points?

How does Skelley figure that a 6-point Clinton win would have her winning 50 GOP-controlled House districts?

His model begins by assuming that the distribution of the national popular vote margin to the congressional districts will be exactly the same in 2016 as it was in 2012.

This may be best illustrated by an example: In 2012, Obama won the popular vote total by 4 percentage points. In Skelley’s model, a 4-point Clinton win would mean she’d also win and lose every congressional district by the exact same amount Obama did.

(Of course, this is bound to be wrong in some places — the distribution of Clinton’s vote share certainly won’t be an exact replica of Obama’s. But since there’s no way of knowing exactly how the popular vote will vary by House district from 2012, it’s a useful gauge of what might happen.)

This is why a 4-point Clinton win probably wouldn’t flip the House. Obama’s 4-point win over Romney gave him victories in just 28 congressional seats currently controlled by Republicans. That wouldn’t be enough — remember, Democrats need a full 30 to take back the House.

But what happens if Clinton expands on Obama’s national vote margin? Since Skelley’s model assumes the national vote share will be evenly distributed, every additional point in the national vote total translates into an additional point for each congressional district. House seats where Democrats tied or lost in 2012 become places where they win at the presidential level in 2016.

If Clinton wins by 5 points, according to Skelley, she’d be projected to win in 39 House seats now run by Republicans. That would theoretically put the House in play but would give Democrats very little margin for error.

But if she wins by a full 6 points, then 50 seats slide over to the Democratic column — at least at the presidential level, and maybe for the House as well.

This isn’t the only prediction that the House really might be in play

Now, Skelley isn’t the only close election watcher who is suggesting the House really might be in play.

Princeton professor and election guru Sam Wang tried to take another stab at this question by looking at how much the Democrats would need to win the popular vote by to win the House, and then trying to figure out where the popular vote currently appears to be headed.

What he found should encourage Democrats. With the exception of just two elections, Wang found that Democrats have always won back the House when they’ve won the "two-party vote" (voting that excludes third parties):

Moreover, the current data suggests that they’re on track to do just that. Wang looked at results from something called the "generic ballot" — it asks voters to pick between the two parties, regardless of candidates — as well as the national presidential polling.

"If the election were held today, House Democratic candidates would win the popular vote by 5 to 8 percent," Wang said. "Judging from the last few cycles, that level of public opinion appears to be right on the edge of being enough to give Democrats control of the House."

The party’s leadership is taking notice. Top Democrats recently told the New York Times that they believe Trump is putting a slew of suburban seats in play that until recently looked like sure bets for Republicans.

"We’d need to win toss-ups or even safer Republican seats, but that’s what happens in a wave: People get sucked in the undertow," California Rep. Adam Schiff told the Times. "And there’s a real chance of that happening thanks to The Donald."

The case to be skeptical that Democrats can take back the House

Still, many of the current projections don’t have the Democrats riding a massive blue wave.

The widely respected Cook Political Report, for instance, thinks the party is only on track to nab an additional 16 seats. The Cook Political Report thinks only 33 seats are vulnerable at all, meaning Democrats would have to win 90 percent of all of the competitive races to pull this off.

Last week, Emory professor Alan Abramowitz released a model for predicting House races based on the "fundamentals" of the race, including the popularity of the two parties and the number of seats at stake. He found that Democrats only have a 15 percent chance of taking back the House.

Similarly, the New York Times’s Nate Cohn argues that there just don’t seem to be enough gettable seats for Democrats. He notes that Democrats’ ability to win back the House in 2006 was made possible by the fact that 21 Republican seats were held in Democratic-leaning districts during that election. There are just nine such seats this year.

"If Democrats are going to retake the House anytime soon, November would probably be their best shot," Cohn said in a story last week, "and as of now it’s not happening."

The big question hanging over this analysis: How many Clinton voters will back congressional Republicans?

There’s an elephant-size question hanging over all the analyses of whether a Trump implosion could give Democrats the House: What happens to "ticket splitting?"

Ticket splitting refers to the voters who back different parties for Congress and the presidency in the same trip to the polls. In 2012, only a record-low 8 percent of voters did so.

If that number goes up in 2016 amid a Clinton landslide, it could give congressional Republicans a buffer from a Trump catastrophe. (Current polling suggests Republican congressional candidates are much more popular than Trump, but it’s unclear how many of the party’s typical voters will actually show up on Election Day.)

This is why the X-factor of Libertarian candidate Gary Johnson makes these kinds of projections much more difficult. Clinton’s lead over Trump in most polls puts her about 6 points ahead, but that’s in part because of how weak Trump is — she’s normally polling around the 47 or 48 percent mark, and he’s closer to 41 or 42 percent.

If Johnson gets in the neighborhood of 5 percent of the national vote, do his supporters vote for congressional Democrats? Or will they vote Republican down ballot — and make Clinton's big national lead less crucial?

This is why Skelley thinks Clinton herself probably needs to win more than 50 percent of the vote to pull Democrats over the finish line. If, for instance, she’s just beating Trump by 48 to 42, then congressional Democrats might need Gary Johnson voters to go blue down ballot, which seems unlikely.

"If she’s only winning barely over 50 percent of the vote, I think it’s difficult to say with confidence that she’ll pull a lot of Democrats with her," Skelley said. "I think she needs to really win the popular vote by more than that."

But if Democrats can’t retake the House with Trump, when can they?

It’s a colossal understatement to say that the stakes are pretty darn high when it comes to control of the House.

If Democrats also go on to win the Senate, winning the House would open the doors for a raft of legislative priorities over climate change, immigration, and a range of other long-held blue sky policy goals for the party. So far, most chatter around a Clinton presidency involves big plans like infrastructure reform and new Supreme Court appointments. Sweeping Congress and the presidency would transform those ambitions into a wholly different kind.

But there’s another, and perhaps equally important, reason this fight is so crucial: It may be Democrats’ last shot at the House for a very, very long time.

New York magazine’s Ed Kilgore explains:

If Democrats do fall short of what they need to regain control of the House even as Hillary Clinton becomes president, prospects for further gains in the near term will probably not be good. The party controlling the White House almost always loses House seats in midterms (1998 and 2002 were the rare exceptions), and 2018 would be a third-term midterm for Democrats, making the odds of an anti–White House trend even stronger. Beyond that, Democrats have a well-known midterm turnout problem associated with their heavy reliance on parts of the electorate — notably young people and minorities — that rarely turn out proportionately in nonpresidential elections. As for 2020, it’s worth noting that Democrats gained only eight House seats when President Obama was reelected in 2012.

Many Democrats have certainly come to despise Trump over the course of a bitter and long campaign. But if he can blunder his way into giving them control of the House of Representatives far earlier than anyone thought possible, maybe they can come to forgive him.