With 18 days until Election Day, Democrats are the clear favorites to take the House, thanks to President Donald Trump's poor approval ratings, a historic number of GOP open seats and record-shattering Democratic fundraising.

But it's not a done deal yet.

Roughly a sixth of America will decide the House. As of Friday, the Cook Political Report rates 75 of 435 seats as vulnerable to a takeover, and it's a lopsided list: 70 are held by Republicans, and only five are held by Democrats. Democrats only need a net gain of 23 seats to win the majority, so winning even two-fifths of these 75 seats would get them there.

But national polls suggest the Democratic enthusiasm advantage has narrowed in the wake of the Brett Kavanaugh Supreme Court confirmation battle. And district-level polling indicates the past month's events have further polarized the House map, with Democrats stretching leads in Hillary Clinton-won seats and Republicans gaining ground in districts won by Trump.

A month ago, a Democratic gain of 20 to 45 House seats appeared possible. The past month hasn't necessarily lowered the Democrats' floor, but it may have lowered their ceiling by limiting their ability to drive their gains deeper into Trump country. At the moment, Democrats could gain anywhere from 20 to 40 seats, with a 25- to 35-seat gain most likely.

I've identified nine types of races that will decide the House's fate. The battleground has expanded since August and spans many different slices of America — from the professional suburbs of Los Angeles to the coalfields of southern West Virginia.

Here's a breakdown of what I estimate Democrats need in each of these nine categories to prevail on Nov. 6:

Vulnerable Democratic Open Seats

If Republicans are going to hold their majority, they'll need to succeed on offense.

Most Democratic incumbents are in excellent shape, but the party holds five at-risk open seats. Of those, they're sure to lose Pennsylvania's 14th District, which Rep. Conor Lamb is leaving to run in a nearby seat after the state's map was redrawn. The next two most vulnerable open seats are in rural Minnesota. Democrats probably need to hold two of five.

Republicans in Suburbs Won by Clinton in '16

Democrats' "blue wave" is most super-charged in elite, professional suburbs where Trump is woefully unpopular with college-educated voters, particularly women.

Ten GOP incumbents are at risk in Clinton-won districts, including areas such as Orange County, California, and the suburbs of Chicago, Dallas and Kansas City.

Republican Rep. Barbara Comstock in Northern Virginia is the most likely to lose. Democrats probably need to flip seven of 10.

GOP Open Seats Won by Clinton

It's not just a "blue wave," it's a "red exodus": There are 41 open or vacant Republican seats, the most since at least 1930. Of those, the most problematic for the GOP are the eight districts Clinton carried in 2016. History is working against the GOP: Since 1992, in situations when a president's party was stuck defending an open seat two years after the president failed to carry it, that party has batted zero for 23 keeping it in their column.

Democrats are almost guaranteed to pick up Pennsylvania's 5th and 6th Districts after the state's Supreme Court redrew them in February to be much more heavily Democratic. One of Republicans' better opportunities to hang on to one of these seats is in Florida's 27th District, a 76 percent Hispanic district where Democrats nominated former HHS Secretary Donna Shalala, who doesn't speak Spanish. Democrats probably need to flip six of eight.

GOP Open Seats Won by Trump

There are 11 more highly vulnerable Republican open seats that Trump carried, ranging from a seat he won by a point (New Jersey's 11th District) to one he carried by 50 points (West Virginia's 3rd District). They include Speaker Paul Ryan's open seat in Wisconsin, and two districts where Republican incumbents lost primaries: Reps. Robert Pittenger's seat in North Carolina and Mark Sanford's seat in South Carolina.

Democrats are virtually guaranteed to pick up New Jersey's 2nd District, where popular moderate Rep. Frank LoBiondo is retiring and national Republicans withdrew support for their nominee upon learning of his comments critical of racial diversity. They're also well-positioned to pick up New Jersey's 11th District, where Appropriations Chairman Rodney Frelinghuysen chose not to seek re-election. Democrats probably need to flip four of 11.

Republicans in Bellwether Suburbs

Ten vulnerable GOP incumbents represent suburban districts Trump carried by a single digit margin in 2016, and most represent a mix of professional and middle-class voters.

Several, like Georgia's 6th District, Illinois' 14th District, Michigan's 8th District and Ohio's 1st District, were gerrymandered to protect Republicans, but political trends have caused the GOP's advantage to unravel. Democrats probably need to flip three of 10.

Republicans in Non-White Districts

Another six Republicans represent districts where majorities of their constituents are non-white.

All but Georgia's 7th District voted for Clinton in 2016. Democrats' challenge is that some of these incumbents are personally popular and Latino turnout typically plummets in midterms. Rep. Will Hurd in Texas's 23rd District is probably in the best shape, in part thanks to his vocal criticism of Trump on immigration. Democrats probably need to flip two of six.

Republicans in "Trump Surge Zones"

Six vulnerable Republicans represent heavily white, working-class districts that previously voted for Barack Obama but swung hard to Trump in 2016. Rep. Rod Blum in northeastern Iowa is the most vulnerable, but Rep. Mike Bost in Illinois and Rep. Bruce Poliquin in Maine are also in serious jeopardy. These are places where the Kavanaugh fight may have boosted the GOP, especially in upstate New York. Democrats probably need to flip two of six.

Republicans in Districts with Urban/Rural Divides

Democrats have pickup chances in 12 districts that combine mid-size cities and large rural components.

In a big blue wave, the risk for Republicans is that energized Democrats in cities like Des Moines, Iowa; Little Rock, Arkansas; and Spokane, Washington turn out at sky-high numbers while voters in red, rural surrounding areas stay home. Reps. David Young in Iowa and Andy Barr in Kentucky are at the greatest risk. Democrats probably need to flip one of 12.

Republicans with Self-Inflicted Wounds

Finally, there are seven Republicans who have no one to blame but themselves for their political woes in Trump-won districts.

They include indicted Reps. Duncan Hunter of California and Chris Collins of New York, whose ruby-red seats wouldn't otherwise be in play. Still, in this polarized political era, Democrats would likely need a tsunami to beat them.

This list also includes Florida Rep. Vern Buchanan, who finalized the purchase of a $3 million yacht on the day Republicans passed their tax package, and Virginia Rep. Dave Brat, who this week told a group of jailed drug addicts: "You think you're having a hard time — I got $5 million worth of negative ads going at me." Democrats probably need to flip one of seven.

David Wasserman, House editor for The Cook Political Report, is an NBC News contributor and senior analyst with the NBC Election Unit.