Analysis by Chris Cillizza

CNN Editor-at-large

(CNN) -- The final week of the Supreme Court session opens Monday, and with it comes rampant speculation that Justice Anthony Kennedy may call it quits.

If Kennedy does announce his retirement, it would almost certainly ensure not only a clear rightward swing in the Court but would also cement a major part of Donald Trump's legacy barely five months into his first term.

The best days Trump had on the campaign trail were all centered on the Supreme Court. His May release of a list of 11 people he would nominate to fill the opening created by the death of Antonin Scalia served as a major rallying point for GOP primary voters still wary of backing someone who was a very recent convert to their way of thinking.

When his campaign was flagging last fall, Trump returned to a familiar strategy -- putting out a wider list of 21 names he might choose to be the next member of the nation's highest court.

And Trump's single best day as president came on February 1 when he announced Neil Gorsuch as his pick to replace Scalia.

Why was the Court Trump's biggest crutch with Republican voters both during the campaign and now? Just ask him!

"If you really like Donald Trump, that's great, but if you don't, you have to vote for me anyway," Trump told GOP voters in Iowa almost a year ago. "You know why? Supreme Court judges, Supreme Court judges. Have no choice, sorry, sorry, sorry. You have no choice."

The point was -- and is -- this: Whether or not conservatives loved or even liked Trump, he would nominate people to the Supreme Court who were considerably more conservative than the people Hillary Clinton would choose. And, the power to nominate justices to the Court mattered more now than ever before because so many of current members were, well, old.

That includes Kennedy who, at 80, is the second oldest justice behind Ruth Bader Ginsburg, who is 84. Stephen Breyer is 78. (For more on Kennedy's pivotal role as the Court's swing vote, make sure to check out Ariane de Vogue's piece here.)

Conservatives have always cared deeply about the federal bench -- atop which sits the Supreme Court -- but a series of rulings over the Obama years made Trump's argument even more powerful. The Court's decision that the individual mandate at the center of the Affordable Care Act was, in fact, constitutional was regarded by many conservatives as a betrayal. The decision to legalize same-sex marriage nationwide was just the sort of legislating from the bench conservatives had long warned about.

If the Court was a major issue for conservatives in the years leading up to 2016, it became the major issue for many of them in the Trump-Clinton race. Sure, Trump said and did things that made them uncomfortable. But what was a little discomfort when compared to decades worth of conservative dominance in the nation's highest court?

A Kennedy retirement -- whether it comes tomorrow, this week or any time between now and 2019 (after that you can be sure Democrats would block any attempt to nominate a new justice, given the Merrick Garland precedent) -- would mean in the eyes of many conservatives that Trump would be the man who changed the legal course of the country. He would become the new Reagan for many in the conservative movement.

That perception would be almost entirely unaffected by any and all of the other things Trump has done or will do over the next three and a half years in office. All his tasteless tweets, his braggadocio, his inconsistent commitment to conservative principles on spending, debt and trade (among other things) would pale in comparison to the fact that he was the Republican president that handed conservatives a solid 5-4 majority on the Supreme Court.

Trump's poll numbers suggest he will face a very tough fight for a second term. His lack of legislative accomplishments could cost him control of the House in 2018. But if Kennedy retires and Trump nominates a reliable conservative justice to replace him, Trump's legacy will be, in no small part, cemented for the very conservatives who held their nose and voted for him.

A Kennedy retirement this week would also make good on Trump's to-date groundless boast that he has accomplished more in a few months than many presidents do in a few years.