Suppose you're Jeff Flake. You have a 2018 re-election bid, but you aren't going to be seriously challenged because despite your state trending a little blue and some demographic shifts happening, you're well liked, the AZ Democratic Party doesn't fully have their shit together, and you won your previous elections by huge margins. You hedged your bet on Trump before the election by nominally supporting the Republican candidate but calling for Trump to withdraw after the tape. You are being primaried from the right for 2018, but it's not serious.
You're 54 years old and you have a solid congressional record with five terms in the house and a full term in the senate. You are in good health and good shape.
You release a memoir / policy manifesto in book format in 2017.
Your ideological background in terms of voting is that you're basically the Median Republican: conservative, but not off with the wing-nuts.
You come out against the Muslim ban. You were part of the bipartisan immigration working group which, by the way, should have worked. You come out swinging with a widely shared op-ed calling for a repudiation of Trumpism and a return to bipartisanship and better relations across the aisle. You are in favour of the employment non-discrimination act, despite being publicly against same-sex marriage. You then very publicly become the go-to guy for a pull-quote on how Republicans deserve to win, but have to do better at working with the rest of the country. You also notably reached out to Gabrielle Giffords when she was shot, and then you were shot at but ultimately not hit by a wacko gunman. In the wake of this, there is wide media reporting about how former president Obama wished you well and that you seem to have a good personal relationship with Democrats and solid, good guy principles. Put it this way -- you're not a Maverick, you're still clearly a Republican, no one is saying you're not conservative, but you seem to be willing to work towards what you think is a better country rather than just laugh and burn it down.
The president (and leader of your party) is extremely vulnerable and unlikely to recover or win re-election. Will he run for re-election? Maybe, but it's not a guarantee.
Are Jeff Flake's actions in the last year consistent with someone who is going to run a primary campaign against the sitting president of his own party? Yes. Are his background and personal characteristics consistent with a good platform to launch such a bid? Yes.