The Supreme Court is wasting no time diving into the thick of 2020 politics. Starting Tuesday, the court will begin debate on whether employers can fire someone for being gay or transgender.

Why it matters: It's the first in a slew of polarizing issues the court will decide over the next 9 months. And Justice Brett Kavanaugh, a polarizing figure himself, will be at the center of it all.

The big picture: The court is slated to hear 3 related cases about LGBTQ rights — one of the specific areas where Kavanaugh's replacement of former Justice Anthony Kennedy may make the most difference.

Federal law prohibits workplace discrimination "because of sex."

The court will consider the cases of 2 men who say they were fired because they were gay, and a trans woman who was fired after she began transitioning.

What they're saying: The employees argue that a person's sexual orientation is inextricably tied to their sex — the employers here didn't have a problem with attraction to men; they had a problem with men being attracted to men.

In the case of the trans employee, her employer was happy with her when she was identifying and dressing as a man, then fired her as soon as she identified and dressed as a woman.

The other side: The businesses' core argument is that the existing law, first written in 1964, does not ban discrimination on the basis of sexual orientation and that Congress could always spell out such a protection if it wanted to.

Between the lines: More than 200 companies — including Apple, AT&T, Facebook, JPMorgan Chase, Starbucks and Uber — have weighed in on the side of the employees. So have big labor unions. The Trump administration has sided with the businesses.

What we're watching: The logic of the employers' argument seems likely to appeal to the court's conservative majority.

Holding that federal law doesn't bar sexual-orientation-based discrimination would be a fundamentally conservative ruling both in its politics and in its deference to the legislative branch. That's a sweet spot for Chief Justice John Roberts.

The employer who fired the trans woman is relying heavily on his religious beliefs, which also tends to find a sympathetic ear at the high court.

The bottom line: This is just the beginning. The court is diving into these polarizing issues quickly and will keep doing so until next summer.

Just last week, the justices accepted a high-stakes abortion case out of Louisiana.

They've also accepted their first big Second Amendment case in over a decade.

They'll hear oral arguments in November over whether President Trump had the authority to end the Obama-era immigration program known as DACA.

Many of these high-profile rulings will come down next June, and it's hard to think of a set of issues — abortion, guns, immigration and the legacy of the Kavanaugh confirmation — that could more fully rile up both parties' bases ahead of the 2020 elections.