Donald Trump just said something that will terrify congressional Republicans

(CNN)Congressional Republicans spend most of their time these days holding their breath and praying that President Donald Trump doesn't say something that will blow up their carefully laid plans for the 2018 election.

Of course, this is Donald Trump we are talking about. And blowing up carefully laid plans is, well, sort of his thing.
Which brings me to Trump's speech at the annual Susan B. Anthony dinner in Washington Tuesday night. Here's part of what Trump said when talking about the coming midterm election:
"But if Democrats gain power, they will try to reverse these incredible gains. These are historic gains. They will try and reverse many of them. So your vote in 2018 is every bit as important as your vote in 2016 -- although I'm not sure I really believe that, but you know. (Laughter.) I don't know who the hell wrote that line. I'm not sure. (Laughter and applause.) But it's still important."
    HA HA HA HA HA ... wait.
    This is vintage Trump. He is reading a prepared speech about how 2018 is so critical because it will determine whether the gains made for conservatives with his 2016 election will be continued in the second half of his first term or not. It's standard issue political speak for a president trying to rally his somewhat too-satisfied base to the necessity of turning out to vote in midterm contests.
    Since there have been midterm elections, presidents have given speeches in the run-up to the campaign trying make sure their bases understand just how high the stakes actually are for this non-presidential year vote.
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    But this is Trump. And Trump never does things the way past presidents (or even past politicians) have. So he reads the line -- "your vote in 2018 is every bit as important as your vote in 2016" -- but them stops himself because, well, he doesn't totally believe it.
    Trump believes that the most important election -- past, present or future -- was his election in 2016. It was the biggest upset in American political history. It was a seminal moment in the ongoing battle between the people and the powerful. It signaled a remaking of not just the Republican party but of how we conceive of -- and analyze -- American politics.
    (Nota bene: I think he's right about all of that.)
    In truth, that doesn't make Trump terribly unique. Every president thinks his election -- especially the first one -- is the most important election. And that the second most important election is their reelection race. The difference? They don't say it out loud.
    Trump's honesty is sure to send a chill down the spines of Republican strategists tasked with trying to hold onto the GOP's 23-seat majority come this fall.
    For Republicans to have any chance at remaining the majority party in 2019, they badly need the Trump base that turned out in droves to vote for the GOP nominee in 2016 to again head to the polls in five months' time.
    The available data since that 2016 election suggests that isn't easy -- even with an engaged Trump. In Alabama, Trump was an active participant in the final days of the special election between Roy Moore and Doug Jones. But he was unable to sway the result to Moore. Ditto a special House election in southwestern Pennsylvania in which Trump campaigned with the Republican nominee, only to watch him come up short with voters.
    What Republicans simply cannot have is a Trump who seems less than jazzed about the 2018 midterms -- and who conveys that "meh" feeling to his core base of supporters. Which is why Trump's comments on Tuesday night have to be so concerning for Republicans.
    Let's be clear: There was a joke-y element to Trump's aside. People laughed -- twice! -- during his 2018-is-no-2016 lines.
    But, in all "jokes" there is an element of truth. Especially when the joker is Donald Trump.
    He often hides behind humor to tell truths he quite clearly believes. Or uses the idea that he was joking to escape criticism or controversy.
    The question Republican candidates and elected officials have to ask themselves this morning is, do they trust that Trump was joking? And, even more importantly: Does Trump's base know that he was joking? And will he make that clear a whole bunch of times between now and the November election? Can anyone make him?
    Even the slightest loss of enthusiasm among Trump base voters could be problematic for the broader Republican Party. Special election results over the past 17 months have confirmed time and again what polls of the electorate also show: That the Democratic base is currently more passionate and motivated to turn out than the Republican one.
    Trump's role over the next five months is to use his personal popularity among Republican base voters to close that gap. But comments like the one he made Tuesday night could well have the opposite effect.