President Trump and Senate Republicans have a … complicated relationship that often changes by the day (or sometimes by the tweet).

But as Trump comes to Capitol Hill on Tuesday to meet with Senate Republicans for the first time in months, there are some remarkably stark divisions between the president and GOP lawmakers, even by the standards of this tumultuous relationship.

Both sides can probably play nice for an hour and put aside most of their differences. (Although you never know. During the 2016 campaign, Sen. Jeff Flake (R-Ariz.) stood up and criticized Trump for saying that Sen. John McCain isn't a war hero.)

Let's review the tensions that senators will have on their minds as they sit down with the president.

McCain is “dying anyway,” day six: Trump's White House has refused to apologize for a remark that Kelly Sadler, a press aide, reportedly made during a conference call last week that dismissed Sen. John McCain's opposition to Trump's CIA nominee because the senator is “dying anyway.”

That has prompted top GOP senators to criticize the White House directly, a pretty rare move. (Normally, it's one or two senators speaking out against the president on any given topic.) McCain (R-Ariz.), who is widely respected on both sides of the aisle, is battling brain cancer. To make light of a Senate legend's illness in such a callous way, and then refuse to apologize publicly for it, is unacceptable to most of the other members of the Senate.

Sen. Lindsey O. Graham (R-S.C.) said that he wished the White House would speak out against an aide who mocked Sen. John McCain (R-Ariz.) during a meeting. (Reuters)

“Obviously, what was said was very wrong and inappropriate. It would have been a lot easier if they had just nipped it right away, and she came out and issued a public apology,” said Sen. John Thune (S.D.), the No. 3 Senate Republican.

An apology would be “appropriate ... from the person who said that really dumb thing,” said Sen. John Cornyn (Tex.), the No. 2 Senate Republican.

“I was confident I was speaking for everybody in the Senate in conveying our deepest respect to him for all he's done for his country during a truly extraordinary life,” said Senate Majority Leader Mitch McConnell (R-Ky.), referring to a recent visit to McCain. He didn't mention the White House, but his message was clear: Don't dis McCain.

“Russian Witch Hunt (there is no Collusion)": There's a difference between professing your innocence and what Trump is doing with the special counsel investigation into Russia interference in the 2016 election. He and his allies are going on a war footing as Robert S. Mueller III's investigation enters its second year. That often means they are trying to undermine it, by calling for it to end or deriding it.

Tweets like the one Trump wrote Tuesday morning are a near-daily occurrence at this point.

That's a problem for Senate Republicans for two reasons:

First, it puts them in the awkward position of having to choose between the president and an independent investigation that is running its course. Most Republicans have tried to split the difference by saying that they think the Mueller investigation should continue unimpeded but refusing to support legislation to prevent Trump from firing Mueller.

Senate Republicans don’t want President Trump to fire special counsel Robert S. Mueller III. They just won't protect Mueller if Trump decides to fire him. (JM Rieger/The Washington Post)

And second, Trump has already strongly considered firing Mueller twice and has given every indication that he might consider it again. For most Senate Republicans, that's a thick red line that no president should cross. If Trump does, they will be faced with a very difficult decision about how to stand up to the president to preserve the integrity of the U.S. judicial system.

Don't forget that Congress isn't done investigating this, either. While House Republicans have wrapped up their inquiry and sided with the president, the less-partisan Senate Intelligence Committee investigation is ongoing. And lawmakers on that panel haven't ruled out the possibility of collusion.

Those tariffs: The disputes we've talked about are mostly political in nature. But Senate Republicans and Trump also have serious rifts on policy, none more dramatic than on tariffs the administration has imposed on a number of countries' steel and aluminum exports.

Mainstream Republicans fundamentally disagree with Trump that tariffs will help the American worker. Rather than restricting imports, Republicans think that open trade will help spur economic growth at home.

Related to this is Trump's about-face over the weekend on trade, specifically in relation to China. He has repeatedly accused China of taking U.S. jobs, but he tweeted Sunday that he is working to save jobs at the Chinese telecom company ZTE, which has been crippled by U.S. sanctions. Trump’s softer approach was a departure from his administration’s tough tactics and caught many Senate Republicans off guard.

Those poll numbers: Trump likes to brag — and did recently as Tuesday morning — about his rising poll numbers, which stand at an average 43 percent. And they are high — for Trump. But his approval rating still is historically low for this point in a presidency.

Why does that matter to Senate Republicans? Because, in a couple of months, a number of them will go before voters in midterm elections that appear to be shaping up as a referendum on Trump's presidency.

That anti-Trump sentiment has helped put Republican-held seats in Nevada, Arizona and Tennessee (and maybe even Texas) in play. Democrats won a Senate seat in a special election in Alabama in December, thanks in part to a flawed candidate whom Trump backed.

All of that has helped put Republicans' slim Senate majority at risk, even though on paper Senate Democrats should be on the defensive.