CNN Special Report "Twitter and Trump" with Bill Weir explores the President's prolific and controversial use of the social media platform Friday at 9 p.m. ET.

(CNN) Republicans should have no trouble not only holding their Senate majority in the 2018 elections but growing it. The party is defending only nine seats as compared to 24 for Democrats -- including 10 in states Trump won in 2016 (and five he won by double digits).

And they still might do just that! But at the moment, it looks like it will be in spite of the Republican president of the United States, not because of him.

Consider Donald Trump's actions in just the last week or so.

On Tuesday night at a rally in Phoenix , Trump didn't mention Arizona Sen. Jeff Flake (R) by name, but he did attack him. "Nobody wants me to talk about your other senator who's weak on borders, weak on crime -- so I won't talk about him," Trump said. "Nobody knows who the hell he is."

Less than 24 hours later, Trump named names -- this time in a tweeted knocking Flake : "Phoenix crowd last night was amazing - a packed house. I love the Great State of Arizona. Not a fan of Jeff Flake, weak on crime & border!"

Then, on Thursday, White House press secretary Sarah Huckabee Sanders called comments made by Tennessee Sen. Bob Corker (R) about Trump's fitness for office "ridiculous" and "absurd." The President himself followed that up Friday morning with this tweet : "Strange statement by Bob Corker considering that he is constantly asking me whether or not he should run again in '18. Tennessee not happy!"

Here's the thing: Attacking Flake -- and seemingly supporting former state Sen. Kelli Ward , Flake's challenger -- makes it less likely that Republicans win Arizona next fall. And outing Corker's retirement dilemma makes it -- stop me if you've heard this before -- less likely that Republicans win Tennessee next fall.

Let's start in Arizona.

Trump carried the state by 3.5 percentage points in 2016 -- and the state has been trending away from Republicans in the last several elections. Flake is a proven vote-getter -- he spent 12 years in the House before getting elected to the Senate in 2012 -- who has almost $3 million in the bank. Ward is a two-term state senator -- she resigned to unsuccessfully challenge John McCain in a Republican primary in 2016 -- who ended June with $84,000 cash on hand.

No matter what you think of Flake or Ward, nominating Ward would be a much larger risk than nominating Flake. She has no record of winning independents and Democratic votes -- she held a Republican-heavy state Senate district -- and no proven blueprint for raising the millions you need to win Senate races.

Meanwhile, Democrats appear to have recruited their strongest potential candidate in Rep. Kyrsten Sinema. ( She hasn't announced yet, but appears headed in that direction .) Sinema has a moderate record, an appealing personal profile and, oh yeah, $3.2 million in the bank as of June 30.

Nominating Ward looks a lot like 2010 when Republicans nominated Christine O'Donnell over Mike Castle in Delaware or 2012 when Richard Mourdock (Indiana) and Todd Akin (Missouri) emerged as their party's Senate nominees. It's no accident that none of that trio is in the US Senate today -- and Democrats now control each of those seats.

Tennessee is less problematic than Arizona for Republicans but still not totally beyond the reach of Democrats. Trump won the Volunteer State by 26 percentage points in 2016, and any Republican nominee -- in the event Corker steps aside -- would be favored to hold the seat.

But with Corker running for re-election, that is a race the national party never has to think about or spend a dime on. Democrats would put up a nominee but almost certainly wouldn't invest in the race.

A Corker retirement, on the other hand, would lead to a likely crowded GOP primary to replace him. And Democrats would do everything they could to convince someone like, say, former Gov. Phil Bredesen to run.

Given that, it makes no sense to antagonize Corker, who has already said publicly he is undecided on whether to seek a third term . Why try to push him off the ledge by revealing private conversations if you know that a Corker retirement has even a 1% chance of complicating Republican efforts to maximize their gains in 2018?

The "why" with Trump is always elusive. My sense here is that Flake and Corker have both raised questions about Trump's presidency and Trump feels compelled to hit back -- regardless of whether it is a good or a bad thing for the broader GOP. (It is, without question, bad for the party.) Trump's first, second and third priorities are Trump. He is marginally concerned -- or interested in -- the fate of the party that he ostensibly leads.

What's far more clear is that Trump is actively hurting Senate Republicans' chances of maximizing their gains in a once-in-a-lifetime election cycle in 2018. Which is a remarkable thing to say about a Republican president.