(CNN) It doesn't matter how farfetched it seems, nor does the math seem to bother him. President Donald Trump continues to be obsessed with Republicans invoking the "nuclear option" in the Senate.

He again goaded Republican leaders to change Senate rules and push his immigration agenda through Congress. (Note: Invoking the "nuclear option," in the overheated parlance of Washington, is using 51 votes to change Senate rules so you don't need 60 votes to cut off debate on bills anymore.) This has already been done for executive and judicial branch nominees, but not for legislation.

Going nuclear is a specific thing Trump wants. Less clear is what piece of legislation, exactly, he'd like to pass.

He's grumbled about the filibuster at least six times as President, saying it's stepped in the way of his agenda on health care, the military and immigration policy.

A sampling:

"With the ridiculous Filibuster Rule in the Senate, Republicans need 60 votes to pass legislation, rather than 51. Can't get votes, END NOW!" he tweeted September 15 as Republicans cast about, ultimately in vain, for a way to repeal the Affordable Care Act.

He said something similar in August of that year.

"If Senate Republicans don't get rid of the Filibuster Rule and go to a 51% majority, few bills will be passed. 8 Dems control the Senate!"

More recently, his rhetoric has changed. He's not talking about health care or budget bills with regard to the filibuster, but rather immigration and his promised border wall. He sounds more urgent, too. He's tweeted three times about the "nuclear option" since January (and twice in two days in April).

April 2: " ...Congress must immediately pass Border Legislation, use Nuclear Option if necessary, to stop the massive inflow of Drugs and People. Border Patrol Agents (and ICE) are GREAT, but the weak Dem laws don't allow them to do their job. Act now Congress, our country is being stolen!

And a day earlier, with even more all caps: "Border Patrol Agents are not allowed to properly do their job at the Border because of ridiculous liberal (Democrat) laws like Catch & Release. Getting more dangerous. "Caravans" coming. Republicans must go to Nuclear Option to pass tough laws NOW. NO MORE DACA DEAL!

Trump has tweeted about the filibuster and the nuclear option at odd times, considering it might not even help him achieve what he's tweeting about.

Republicans couldn't get the simple majority for their Obamacare repeal effort in the Senate under budget reconciliation rules.

And as CNN's Manu Raju pointed out, Trump's own immigration plan got only 39 votes when it came up in the Senate recently. The nuclear option wouldn't have helped that bill. Not one bit.

As Trump today and yesterday renews call for nuclear option lowering threshold to clear filibuster to a simple majority, reminder that his own immigration plan failed with just 39 votes in February. https://t.co/w5LVhaxX4Y — Manu Raju (@mkraju) April 2, 2018

Senators end up doing a lot of simple math, as trying to get to 60 votes on anything is extremely difficult, which is a large part of the reason so little seems to get done on Capitol Hill.

No Democrat is going to want to sign on to giving away their only real means of standing in the way of Trump and his agenda. And for Republicans who might want to just do away with it, it's important to remember that a Democrat, Harry Reid, first invoked the nuclear option in 2013. It was a slippery slope to Republicans, under Majority Leader Mitch McConnell, using the nuclear option to confirm Neil Gorsuch to the Supreme Court last year.

Every single one of the 51 current Republican senators would have have to sign on to the idea of changing Senate rules with a simple majority.

It's just not going to happen in the current Senate. Why? There's a filibuster-proof majority of senators -- 61 of them! -- who went on the record in April 2017 and signed Maine Republican Sen. Susan Collins' letter to Senate leaders asking them not to change Senate rules any more.

When Trump floated the idea of invoking the nuclear option back in January, a spokesman for McConnell repeated that he had no intention of doing so.

All that said, from an expansive historical view, senators have been slowly checking their own abilities to stymie legislation and appointments for decades.

The Senate historical office charts some of the inflection points , including a threat in 1841 to change Senate rules that allowed for unlimited debate. The change to rules in 1917 that created the modern filibuster but required 67 votes to do so. The ugly 60-day filibuster of the Civil Rights Act in the 1960s. The changing of Senate rules to make it easier to cut off debate with only 60 votes in 1975.

You see where this is going, right?

It might not happen today or tomorrow, but when the rules were changed by Democrats to get more judicial nominees confirmed for President Barack Obama, that's when the Senate most recently went nuclear. And we've been living in a post-nuclear Senate world since 2013. Republicans carried on the tradition by going nuclear to get Gorsuch confirmed. The power is there and senators have used it. It's not crazy to think that someday, perhaps soon, when the majority party really wants to pass something, they'll change the rules to do it. Unfortunately for Trump, the bills he's groused about haven't even had a simple majority. Also, the Senate changes very slowly. Until it doesn't.