Unless the trajectory of the race changes, the Senate is highly likely to flip. Mark Kirk and Ron Johnson are almost certain goners and Pat Toomey and Kelly Ayotte both trail narrowly right now. The last poll of North Carolina also had Richard Burr down by two points. Assuming Hillary wins, Democrats would have Tim Kaine as the tiebreaking vote if they take four of those five races (unless the GOP flips Harry Reid’s seat to red in Nevada, that is). With all five, they wouldn’t need Kaine. Which means it’s very, very important that Republicans maintain their majority in the House. If that flips too, the only congressional check on President Hillary until 2018 would be Chuck Schumer’s willingness to protect the filibuster in the Senate. You trust Chuck Schumer, don’t you?

So how likely is the House to turn over? Not very, but Chris Cillizza ran some back-of-the-envelope numbers based on a guesstimate by a Republican pollster that the average House candidate can, realistically, run no more than five points ahead of Trump in his home district. Some voters will split their tickets this fall, voting Clinton or third-party for president and then Republican downballot, but you can’t reasonably expect more than five percent or so to do that in a House race. Which, if true, means that Republicans potentially have a problem in any district where they enjoy less than a five-point registration advantage. Cillizza:

The Cook Political Report lists 45 Republican seats and 11 Democratic ones as potentially competitive this November. Let’s focus in on those 45 Republican seats.

Of the 45, 40 remain largely intact from the 2011 national redistricting process. (Florida engaged in a mid-decade redraw.) For those 40 seats, we can overlay the Cook Report’s Partisan Voting Index (PVI) over them in an attempt to compare apples to apples. (The PVI ranks every district against every other district based on presidential performance.) Of the 40 GOP-held districts, 36 of them have a PVI of R+5 or lower, meaning that they are five points (or less) more Republican at the presidential level than all of the other districts in the country.

If Republicans lost all 36 of those seats with a PVI of R +5 or lower — and Democrats held all 11 of their contested seats — Democrats have the House majority. By six seats. Twenty seven of those 40 seats have a PVI of R+3 or lower. Win those 27 and Democrats need only to pick up three seats among the slightly more Republican-friendly districts to win the majority.

Jay Cost was thinking along the same lines this morning in noting that, among Colorado, Virginia, North Carolina, and Florida, Team Trump is within five points in just one state right now — and it’s by exactly five points. Needless to say, the bigger Clinton’s lead gets, the more even some fairly solidly red districts will be in play:

The good news for the Senate is that ticket-splitting might be more common there because senators tend to have a higher public profile than congressmen do. You might not know who your rep is in New Hampshire or Pennsylvania but you probably know who Kelly Ayotte or Pat Toomey is. In the case of senators who have run for national office, like Marco Rubio, that’s even more true. The average voter may be more inclined to vote Ayotte, Toomey, or Rubio for Senate even if they’re voting for Hillary for president than they are to vote for their Republican House incumbent. And yet, even that has limits. Toomey right now is much more competitive than Trump is in Pennsylvania, trailing Katie McGinty by an average of 2.6 points versus the 9.2-point margin Trump is starting at. That’s a nearly seven-point difference. (Although all of it comes in Clinton outperforming McGinty. Toomey and Trump are polling nearly identically, an ominous sign.) He’s still losing, though. No incumbent, including Rubio, is so sturdy that he or she can’t be dragged under by a rising blue tide. Harry Enten of FiveThirtyEight ran the numbers and concluded that, yes, the blue tide is rising on the Senate:

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With the Senate slipping and the House increasingly in jeopardy, normally this would be the point when the RNC huddles and starts making hard decisions about which races to spend its money on. Because Trump isn’t a loyal party soldier, though, they can’t count on him to keep fundraising for the downballot races or even to encourage his hardcore fans to turn out if the RNC pulls the plug on him and he fades decisively. Having anchored themselves to Trump, the only remaining hope the party has of retaining power in Congress is to keep him from sinking. He has to do well at the debates and he, or some third-party like Wikileaks, has to do serious damage to Clinton to make the race competitive. Simple as that.

Here are Ayotte and Rubio trying their best to walk the line between embracing Trump to stay on the right side of his fans and crossing Trump to stay on the right side of swing voters. They’re both supporting him, but they both also vow to act as a check in the Senate on whoever the president ends up being next year. Think voters will buy it?