A read well worth your time from Sean Trende on another slow-except-for-Ukraine news day. How does he come up with a probability like that? By using an indicator that’s helped predict midterm elections in the past — namely, the president’s job approval on election day — and then running simulations a la Nate Silver for various different approval ratings to see how many seats the GOP could expect to gain (or even lose).

The result: Democratic seppuku.

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The Senate’s 55/45 right now, which means Obama’s approval rating could rise as high as 49 percent and the GOP’s low-end expectation would still be six seats, flipping the chamber. According to Gallup’s latest job approval figures today, O’s back to 46 percent — the highest he’s been in ages but still a number that could plausibly produce a double-digit pick-up for Republicans. In fact, just to put in perspective for you how heavy a lift this is for Democrats, here’s RCP’s graph of Obama’s job approval for the past two and a half years, since July 2011:

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The only time he reaches 50 percent is right after election day 2012. He managed to stay above water for a few months afterward, during his post-election honeymoon period, but he was break-even at best before then and, starting with the Snowden revelations last summer, lately it’s been nothing but a steady decline. All of which is to say, barring an amazing turnaround for ObamaCare or some sort of unforeseen crisis a la Christie’s handling of Hurricane Sandy, there’s no reason to expect O’s job approval to soar over the next nine months. It would defy more than two years of precedent. There’s no historical reason to expect it either; there was a rash of articles last year after Obama’s numbers started to crumble noting that once a second-term president’s job approval lands in the crapper, it usually doesn’t make it out again. Maybe O’s rating will inch up a bit, or maybe it won’t once the next round of ObamaCare chaos begins. Given that he’s had a ceiling of about 48-49 percent since mid-2011 (again, except for his election honeymoon), which way would you bet? Follow the link up top and scroll down to Trende’s bell-curve graph of likely Democratic losses and you’ll see there’s a greater than 80 percent chance that they lose six seats or more. In theory, at least: In practice, it may be that O’s low approval rating makes a blue-state Senate race surprisingly tight but doesn’t quite push the Republican challenger over the top against the Democratic incumbent.

One more thing. Trende’s last piece of election forecasting noted that the 2016 Senate cycle looks to be as difficult for Republicans as this one is for Democrats, meaning that the GOP needs to pile up wins this November to buffer itself against heavy losses two years from now. According to that forecast, Democrats would have a better than even chance of retaking the Senate in 2016 if the GOP wins eight seats or fewer this time even if a Republican takes the White House in 2016. That means the GOP needs to win nine this year to be somewhat safe, and according to Trende’s table above, that could require an Obama approval rating of around 44 percent on election day. Obama’s approval rating today, per the RCP average: 44.4 percent. Hmmmmmmm.