Analysis by Chris Cillizza, CNN Editor-at-large

(CNN) -- House Speaker Paul Ryan spent the past two years drawing lines in the sand regarding President Donald Trump -- and then erasing them when Trump, inevitably, overstepped.

In May 2016, Ryan said he couldn't endorse Trump -- even though the businessman was the presumptive nominee -- because he needed to show he could unify the party. Trump didn't.

The next month, Ryan said Trump's comments about Judge Gonzalo Curiel amounted to "the textbook definition of a racist comment." Trump didn't apologize.

The most prominent "break" was in early October 2016 in the wake of the "Access Hollywood" tape that showed Trump bragging about sexually assaulting women. Ryan pledged that he would no longer defend Trump or advocate for his election. "The speaker is going to spend the next month focused entirely on protecting our congressional majorities," a Ryan spokeswoman said at the time.

You know how that one ended. Trump won. Republicans -- including Ryan -- celebrated their newfound control of both ends of Pennsylvania Avenue. "This is the most incredible political feat I've seen in my lifetime," Ryan said shortly after the election.

Which brings us to Sunday and Ryan's latest line in the sand for Trump: Don't fire special counsel Robert Mueller.

"We need to let these career professionals do their jobs, see it through," Ryan said on "Fox News Sunday. "So, no, I don't think he should be stepping down, and I don't think he should be fired. And the President has made it clear, he's not going to do that."

The message is clear: Firing Mueller would be too much, a stepping beyond the bounds by Trump. And, if the President did take that course of action, Ryan -- and presumably the Republicans loyal to him in Congress -- would have no choice but to disavow Trump for good.

Now, Ryan is generally right that Trump has said of late that he has no plans to remove Mueller from his job -- even as one former Trump adviser had pleaded guilty to lying to the FBI about his interactions with foreign officials close to the Russian government and two others have been indicted for alleged money laundering.

"Well, I hope he's treating everything fairly," Trump told journalist Sharyl Attkisson in an interview over the weekend. "And if he is, I'm going to be very happy, because when you talk about innocent, I am truly not involved in any collusion with Russia."

Obviously, there is plenty of wiggle room in that statement -- "fairly" is in the eye of the beholder -- but all of the reporting coming out of the White House suggests that Trump has rejected -- for now -- the hardline approach to Mueller advocated by, among others, former chief strategist Steve Bannon.

The truth is that no matter what Trump does with Mueller, the history of Ryan's line-drawing should make anyone paying attention skeptical that this time he really means it.

While Trump's poll numbers with the general public are as bad as they've ever been, he remains very, very popular with people who voted for him. A new Washington Post-ABC News poll found 91% of Trump voters approve of the job he is doing, with 69% strongly approving.

In raw political terms, that means there is still considerable peril in openly breaking with Trump. One needs only look as far as Arizona Sen. Jeff Flake, a Republican whose polling numbers collapsed after he wrote a book earlier this year taking Trump -- and the GOP -- to task. Flake decided to retire last month, a decision he all but acknowledged was forced upon him.

Which means that Ryan breaking with Trump even if Trump fires Mueller would be politically perilous for the speaker. And politicians -- especially in this day and age -- very rarely do things that they know could cost them their jobs.