One of the simplest explanations for Republicans majorities of nearly every level of government is President Obama. Voters like a check on the party in the White House, and the check they put on Obama was particularly strong.

So it stands to reason that the reverse is probably true: A President Donald Trump should be a boon for Democrats down ballot. And if the traditional rules of politics hold, it probably will be. But for Democrats, it is not as simple as putting a Republican in the White House and hoping for the best.

A variety of factors could ensure that even with a President Donald Trump, Democrats remain in the minority for years to come. Let's run through them:

One number: 2020

Some results are still coming in, but it looks as if in 2017, Republicans will continue to control up to 69 of 99 state legislative chambers. And controlling state legislative chambers is one of the keys to controlling government. Once a decade, in most states, lawmakers get to draw the very lines that elect state and congressional lawmakers.

In theory, lawmakers are recalibrating their districts to better reflect new census data. In practice, many lawmakers also take this redistricting opportunity to rejigger the lines to benefit their parties for the next decade.

As Republicans showed in 2010, investing in state races is one of the best bangs for your buck to swing statehouses, and Washington, the way you want.

I explained how in July:

Fueled by millionaire Art Pope, Republicans invested $30 million into winning state legislature battles, particularly states where the legislature was up for grabs, and it had a say in redistricting.
The next election, Republicans picked up 675 state legislative seats. That meant the GOP in total controlled about three times as many states in the redistricting process, including many big, swing-y states where the lines are even more fungible and important.
New lines were drawn, and in 2012, Republicans took over the House of Representatives with a commanding 234-201 majority — despite the fact Democratic House candidates got 1.4 million more votes than Republican candidates.

Democrats are trying to outmaneuver Republicans by creating a redistricting-focused group — led by former U.S. attorney general Eric H. Holder Jr., backed by Obama and filled with some of the smartest minds in Democratic down-ballot politics — dedicated to winning back seats by 2020. (In 2014, Democrats launched Advantage 2020, a super PAC that hopes to raise $70 million to play exclusively in states where redistricting is on the line.)

But their efforts may come too late for this next redistricting battle. They have got only two election cycles — 2018, 2020 — to catch up before it is time to redraw the maps for the next 10 years. And Democrats are in such a big hole that it may take even more time to rebuild their majorities in state chambers, which means they could be locked out of the redistricting process in some key states for another decade.

The next U.S. Senate map is brutal for Democrats

November was a tough election cycle for Senate Republicans, who were defending 24 of the 34 seats up for grabs, many in states that Obama won twice.

It will basically be the reverse in 2018. Democrats are defending 10 seats in states that Trump won, sometimes by double-digit margins. Midterms are normally kind to the party not in power, but this map shows serious head winds for Democrats.

Democratic Sens. Joe Manchin III (W.Va.), Joe Donnelly (Ind.), Claire McCaskill (Mo.), Heidi Heitkamp (N.D.) and Jon Tester (Mont.) are running for reelection in states that voted for Trump over Clinton by 19 points or more. (In West Virginia, Trump won by 42 points.) If these Democratic-held seats and a few others fall to Republicans in 2018, it's possible the GOP's 52-seat majority becomes a 60-seat supermajority. At the very least, it looks likely Republicans will pick up a few seats.

More bad news for Democrats: Some political analysts think that if Republicans turn seats in red or red-leaning states, such as West Virginia, Indiana and Missouri, those seats could stay Republican for a long time. Especially if 2016's presidential election is any indication.

My colleague Philip Bump calculated there were 27 counties that had supported the Democratic candidate consistently for at least 40 years that switched to Trump in this year's election. Those counties were clustered in states such as Indiana and Michigan, where Democrats will be defending Senate seats.

Democrats have a lot of ground to make up — but they lack the troops to take that terrain

To drive that point home, let's run through the partisan control of 2017 again:

  • Republicans will control 68 or 69 of 99 state legislative chambers, a 2-to-1 advantage over Democrats.
  • They will control at least 33 of 50 governors' mansions, and Democrats will control 15 (or 16 if they hang onto a slim lead in North Carolina's governor's race). Winning North Carolina would be impressive, but Democrats are still 10 seats away from a majority of governors' mansions. (Though Democrats are actually looking forward to the 2018 map, where Republicans will be defending 12 seats in states that Obama won twice.)
  • Republicans will control 238 of 435 congressional seats, a 24-seat deficit for Democrats.
  • And Republicans control 52 of 100 Senate seats, a number that could grow in 2018.

At almost all levels of government, Democrats will have to claw back from a double-digit deficit to regain majority control. It's certainly possible, and they will be aided by a Republican in the White House just as Republicans were aided by Obama to gain their majorities.

But it's not going to be easy, or quick — especially because Democrats have spent the past eight years losing and therefore losing the talent that will help them claw back.

“If you don't have state legislators, you don't get a good choice of candidates for governor, for Senate and House and for the president,” said Louis Jacobson, a nonpartisan state race handicapper. “So the longer you go with a weak team down-ballot, the harder it gets to climb out of that hole.”

Lack of time, a dearth of talent and a little bit of bad luck with the Senate map could mean Democrats stay in the hole for years to come.