(CNN) It makes all the sense in the world that President Donald Trump would personally go after Sen. Jon Tester.

Mysteriously and without any proof, Trump said he knows things that could sink the Montana Democrat's re-election bid. We assume this is a specious claim from a master conspiracy theorist until we see some sort of proof.

That aside, it makes every bit of sense that Trump would want to go after Tester.

Yes, Tester picked the fight, of sorts, as the top Democrat on the Senate Veterans' Affairs Committee, becoming the public face of opposition to Rear Adm. Ronny Jackson's since-withdrawn nomination to be veterans affairs secretary

Start with that, then add to it that Tester is up for re-election as a Democrat in Montana, one of just 12 states where the President's approval rating has been over 50%, according to data released by Gallup back in January, the most recent available.

His national approval rating has hovered around 40%. In Montana, it was 52%.

Trump won Montana with 56% of the vote in 2016. It's one of five states with Senate Democrats up for re-election that the President won by double digits.

And this is also the difficulty for Democrats heading into midterm elections in November in which they have very high expectations and scads of momentum.

They are expected to do well, but they also face the hard reality that 26 of the 35 Senate seats being contested are already held by Democrats. There aren't many places they can go on offense.

Trump made clear the beef was about more than the VA nomination and Jackson Monday night on Twitter when he accused Tester of supporting an "open borders agenda" and somehow found a way to tie Tester to the migrant caravan Trump has been tweeting about

FiveThirtyEight has a clever way of measuring how politically suited a senator is to a state, at least when it comes to support for the President. It compares the President's margin of victory in 2016 with how consistently a senator votes with the President. They call it the "Trump plus-minus," and Tester has the biggest negative plus-minus, at -48.4 (Tester voted with Trump 37.1% of the time, but Trump won the state by 20.4 points. For comparison, the Republican with the biggest positive plus-minus is Sen. Cory Gardner, a Colorado Republican who is not up for re-election this year.

JUST WATCHED Montana voters weigh in on Trump/Tester feud Replay More Videos ... MUST WATCH Montana voters weigh in on Trump/Tester feud 03:50

Nine of the top 10 Democrats on the plus-minus list are up for re-election. Only one Republican is. CNN rates seven Senate races in the "Toss-up" category -- 5 seats currently held by Democrats and only two held by Republicans.

To take control of the Senate, Democrats would have to keep all 26 seats they currently hold and pick up two more. We've written in the past, repeatedly , about the difficulty of that task when they are defending seats in states like Montana and North Dakota, and there simply are not many Republican-held seats in Democratic territory being contested this year. Or they'll have to make inroads in what has heretofore been deep red territory. They did that with the special election in Alabama in December, when Democrat Doug Jones was elected. But that took an incredibly flawed candidate like Roy Moore, who essentially imploded during allegations that he had pursued and, in some cases, sexually abused teenage girls when he was in his 30s.

Tester has survived as a Democrat in a red state, and he's been elected to the Senate there twice before. Montana has remained in the "Lean Democratic" column in CNN's Key Race ratings, in large part because Republicans there have not yet found a top-flight challenger to take on Tester. The senator first won election in the Democratic wave year of 2006 and was re-elected in 2012, a presidential election year when Republican Mitt Romney carried the state with 55% of the vote, 13 percentage points more than then-President Barack Obama. Tester won by 4 percentage points, with 49% of the vote.

Trump is now targeting Tester, in a state where they are both popular. Will Trump's attacks make Montanans rethink their support for Tester? Whether red states like Montana still want blue senators is going to be one of the main political stories between now and November.