Democrats remain the favorites to take back the House in November, but their already long-shot odds of retaking the Senate have gotten dramatically worse in recent weeks. Senate races in North Dakota and Tennessee have slipped away from them, and even battles that Democrats have been favored to win for months are tightening up or even leaning toward the Republican candidate.

Democrats look like they have about a 1 in 5 chance of seizing the Senate. Those are roughly the odds of hitting your flush draw on the final card in a game of Texas Hold 'Em: It's not impossible, but it's pretty unlikely.

Failing to take back the Senate would be painful, yes, but behind the 2018 Senate races lurks a 2020 map that isn't great for Democrats, either. That means that holding on to existing seats is nearly as important as gaining new ones: If Democrats want to have any chance of controlling the Senate before 2023, they'll need to stay within striking distance.

The 2018 Senate map is legendarily bad for Team Blue, which is defending 10 seats in states Trump won in 2016, many by landslide margins. In Tennessee, Democrats recruited popular former Gov. Phil Bredesen to vie for the seat of retiring GOP Sen. Bob Corker. Bredesen led the polling for much of the spring and summer against far-right Rep. Marsha Blackburn. But Bredesen appears to be experiencing the same fate as former Indiana Sen. Evan Bayh in his 2016 Senate race, where the underlying dynamics of a very conservative state are leading voters to choose their partisan identity over a centrist Democrat from the recent past.

Tennessee might also be among a handful of places where the Brett Kavanaugh hearings really hurt the Democrats: While Bredesen went on record saying he would have voted to confirm Kavanaugh, the ugly confirmation fight might have made local Republicans less likely to cross over to vote for any Democrat.

This also seems to be the case in North Dakota, where incumbent Democrat Heidi Heitkamp was already trailing her Republican challenger Kevin Cramer. Heitkamp voted no on Kavanaugh, and has released ads explaining her vote. But the damage is done. Like Bredesen, Heitkamp now looks like a heavy underdog.

And in Nevada, incumbent Republican Dean Heller is running a much stronger-than-expected campaign against his challenger, one-term Democratic Rep. Jacky Rosen. Like a number of Senate Republicans' races, Heller's fortunes seemed to improve after the Kavanaugh hearings, but that momentum has stalled out, with Rosen narrowly leading in the last two polls. Heller, who famously announced his opposition to the GOP health-care bill in the summer of 2017, and then in the end folded and voted for the doomed effort to repeal ObamaCare, was thought to be the most vulnerable Senate Republican this cycle, but right now this contest looks like a pure toss-up.

In Arizona, where Democrats had long been hoping to pick up the seat of retiring GOP Sen. Jeff Flake, Democratic Rep. Kyrsten Sinema now finds herself in a dead heat with Republican Rep. Martha McSally. Republicans were lucky that pardoned criminal Joe Arpaio and Rep. Kelli Ward split the fringe vote in the primary, delivering the party its best possible candidate in McSally, a former fighter pilot and, at least in the context of today's Republican Party, something of a moderate. The GOP's attacks on Sinema's past as an anti-war activist are working. Meanwhile, Sinema has taken great pains to emphasize her moderate voting record, calling herself the "third most independent" member of Congress and promising to work with President Trump when the situation calls for it. Whether that strategy succeeds in goosing the kind of turnout Democrats need to win in Republican-leaning Arizona remains to be seen.

There is simply no plausible path to a Democratic Senate majority that doesn't run through successful campaigns in both Nevada and Arizona. And if Heitkamp is really a goner in North Dakota, Democrats will need Tennessee or Texas, too. Texas Democrat Beto O'Rourke is a sensation, but he's down by an average of 7 points to Republican Sen. Ted Cruz in recent surveys. And that's to say nothing of close races in Missouri and Florida. Incumbent Democrat Claire McCaskill is tied in polling averages with Republican Josh Hawley, and in Florida, the flagging campaign of Democrat Bill Nelson may be getting a lift from gubernatorial candidate Andrew Gillum, a potential national star. Nelson looks like he has a small lead, but those races could still easily go either way.

So, things are not looking good! But those hoping for a reprieve from this kind of steep incline have probably not had a look at the climb facing the party in 2020.

Only Sens. Susan Collins (R-Maine) and Cory Gardner (R-Colo.) occupy Republican-held seats in Clinton states, and neither will be easy to unseat in 2020. Democrats also have to defend the Alabama seat improbably captured in December 2017 by Doug Jones, which will be a struggle against any Republican who managed not to seduce teenagers in his 30s.

But there are a few seats that might be in play if the national environment remains as bad for Republicans as it is today, or if the economy goes into a long overdue recession — including those of Joni Ernst in Iowa, David Perdue in Georgia, and Thom Tillis in North Carolina. And Democrats will get another crack at an Arizona Senate seat, because Jon Kyl's abbreviated term as fill-in for the late John McCain will be expiring. But if that's the landscape of flippable targets, Democrats may very well be underdogs everywhere but Colorado.

Don't get me wrong — taking aim at a series of Republican-leaning purple states will be easier than this year's task of turning Tennessee or Texas blue, but it still won't be easy. Particularly in Georgia, Iowa, and North Carolina, Democrats will face the additional hurdle of the GOP's rancid voter suppression machine, which is in full swing right now against Georgia Democratic gubernatorial candidate Stacey Abrams.

Are there any races that might be unexpectedly competitive? There's a lot of anger on the progressive left about Lindsey Graham's (R-S.C.) recent transformation into a raving Men's Rights Activist and shameless surrogate for the president. But he won his race in 2014 by nearly 16 points, and there are probably just not enough Democrats in South Carolina to take him down. The same goes for Senate Majority Leader Mitch McConnell in Kentucky, who blew out a strong challenger in Allison Lundergan Grimes in 2014. And if the state's most popular Democrat can't pick up the open seat in Tennessee this year against hardliner Marsha Blackburn, they probably have little hope of defeating longtime Sen. Lamar Alexander in 2020 if he runs for re-election.

Realistically, that means Democrats can't dip below 47 or 48 seats after this November's elections without being overwhelming Senate underdogs again in 2020. So while O'Rourke's phenomenal campaign in Texas has energized the national left, progressives are likely to have a greater late-game impact by volunteering on behalf of Sinema, Rosen, McCaskill, Heitkamp, and Nelson.

Barring some radical change in his personality that leads him to start speaking and appealing to the majority of Americans who voted against him, Trump will begin the 2020 presidential campaign as a uniquely vulnerable incumbent, doubly so if the economy flags in the next 18 months. But taking the presidency and the House in 2020 while facing a hostile Republican Senate would cripple the next Democrat in the White House in the same way that Barack Obama saw his legislative dreams die when Paul Ryan took the speaker's gavel from Nancy Pelosi in 2010, and then Mitch McConnell's Republicans captured the Senate in 2014. And it won't be just progressive policy that dies in McConnell's clutches — Democrats also won't be able to use unified government to grant statehood to D.C. and Puerto Rico, or pass national legislation to address escalating GOP voter suppression. Each failure to capture power will, depressingly, make it harder to win the following cycle.

That is not to say all is lost for Senate Dems on Nov. 6. For starters, evidence of the so-called "Kavanaugh bounce" touted by conservatives is sketchy at best. Where is that bounce for Republican Senate challenger Mike Braun in deeply conservative Indiana, where incumbent Democrat Joe Donnelly is in better shape than he was in August? Why did Kavanaugh help Dean Heller but not Rick Scott in Florida? Why have Democrats' numbers on the generic ballot held steady throughout the tempest?

Additionally, a lot of the pessimism hinges on a decline in Democratic fortunes in the North Dakota and Tennessee races, both of which have been pretty sparsely polled considering their importance. It's also worth remembering that Heitkamp trailed late in her 2012 Senate race too, only to see a Democratic turnout surge deliver her into office.

There's still time, in other words, for the fundamentals of this election — the toxicity of the president and the unpopularity of the Republican legislative record — to reassert themselves, even in tough races.

While Democrats were never particularly close to being favored to take the Senate by forecasters and odds-makers, an upset victory would nevertheless be particularly sweet. But they have to consider the long game, not just 2018. A Democratic House and Senate would not be able to make meaningful policy with Trump in the White House. And that means that maintaining their chance of flipping the Senate in 2020 should be just as important a goal for Democratic strategists.