Almost two hours into a blustery GOP debate Thursday night, Sen. Ted Cruz (R-Tex.) and Sen. Marco Rubio (R-Fla.) went down a rabbit hole.

As they exchanged barbs about who's tougher on immigration, the two went deeper and deeper into the inner workings of Congress, bickering about who voted for whose amendment and who said what on the Senate floor and when.

A sampling:

Rubio: Ted Cruz, you used to say you supported doubling the number of green cards, now you say that you're against it. You used to support a 500 percent increase in the number of guest workers, now you say that you're against it. You used to support legalizing people that were here illegally, now you say you're against it. You used to say that you were in favor of birthright citizenship, now you say that you are against it.

Cruz: Marco stood with Chuck Schumer and Barack Obama on amnesty. I stood with Jeff Sessions and Steve King. Marco stood today, standing on this stage Marco supports legalization and citizenship for 12 million illegals. I opposed and oppose legalization and citizenship. And by the way, the attack he keeps throwing out on the military budget, Marco knows full well I voted for his amendment to increase military spending to $697 billion.

Who supported what with regard to immigration reform is a fair question for any one of the seven candidates on stage in Thursday's sixth GOP debate. Nearly all of them have contorted their positions to some degree.

But this argument continues to center around Rubio and Cruz more than any of the other five candidates on stage Thursday for one main reason: They're in Congress.

And that's one of the toughest jobs for anyone running for president. Politicians say things, then back out of what they said, all the time. But for members of Congress, it's literally their job to put what they think in the public record.

When Cruz and Rubio and the other two senators in the race -- Rand Paul (R-Ky.) and Bernie Sanders (I-Vt.) -- are not campaigning, they're in Washington voting on resolutions and bills and amendments to those bills. And when they're not on the Senate floor voting or debating, they're in committee hearings, talking about the issues. Or sometimes giving speeches. In between it all, they're constantly tracked by reporters, who have free rein in Capitol halls.

Basically, there's a lot more of their records to pick apart than other other politicians they're running against -- including governors who have significantly fewer concrete proposals come to their desks.

And what they say is instantly fact-checkable, as Twitter made clear throughout the night.

In a presidential race, a member of Congress's record is opposition research just sitting there for the taking. And as the race for everyone below Trump heats up, Cruz's and Rubio's opponents are digging through every bit of it.

Former Florida governor Jeb Bush's super PAC, Right to Rise, produced an ad comparing Rubio and his votes in the Senate to a weather vane, blowing whichever way the politically opportunist wind goes. At the end, a train rolls by with a photo of Bush, who the ad says, is the picture of consistency.

New Jersey Gov. Chris Christie used a similar line of attack when he got into it a bit earlier with Rubio:

"See when you're a senator, what you get to do is just talk and talk and talk," Christie said. "And you talk so much that nobody can ever keep up with what you're saying is accurate or not. When you're a governor, you're held accountable for everything you do."

It's also worth noting here that President Obama is the first person since John F. Kennedy to be elected directly from the Senate.

And if lawmakers running for president are more vulnerable to attacks, they also arguably have a more difficult time defending themselves from those attacks. Making laws is an arcane process with procedures and language literally from another century. The process can be opaque and confusing. Explaining why they voted a certain way into a succinct soundbite is difficult.

In short, being in Congress is complicated -- especially in a Republican Party these days that hates conservative impurity.

But we'll end by noting the biggest reason members of Congress who are running for president are in an unenviable position is actually quite simple: Their employer isn't very popular. And especially in this election cycle, that's reason enough.