With Attorney General Jeff Sessions appearing before the Senate Intelligence Committee on June 13, there's a lot lawmakers want to straighten out. Here are three of the major questions they'll have. (Jenny Starrs/The Washington Post)

Attorney General Jeff Sessions could be at the center of two controversies in the Trump administration: whether Donald Trump's campaign colluded with Russia to help Trump win and whether the president obstructed justice.

That's why it's a big deal that he'll testify Tuesday before the Senate Intelligence Committee, the main committee in Congress investigating Russian meddling in the campaign and potential Trump meddling in the fallout.

Here's why Sessions is at the center of so much, and here's how he can help us better understand the still-unraveling Trump-Russia-FBI investigation.

1. He met with Russians during the campaign, when they were allegedly trying to help Trump win

Jeff Sessions prepares to testify in January at his confirmation hearing to be attorney general. (Jim Lo Scalzo/European Pressphoto Agency)

Besides Trump's son-in-law, Jared Kushner, Sessions is the highest-profile member of Trump's campaign and administration who we know met with Russians during the 2016 campaign and didn't disclose doing so. Sessions didn't disclose those meetings when he was asked, under oath, during his confirmation hearing.

A day after The Washington Post reported that, Sessions recused himself from the Russia investigation.

Why that's a big deal: It's normal for U.S. political campaigns and foreign officials to talk. But former CIA director John Brennan recently told Congress that his “radar” went off anytime Russians met with the Trump campaign because he knew the Russians were trying to influence the election, and he knew they often did that by trying to recruit “either wittingly or unwittingly” U.S. officials to help.

What Congress will want to know: A lot. Are there more meetings Sessions had that he didn't disclose? Why isn't he forthcoming about these meetings? What did Russia want to talk about? Did he get the feeling Russians were trying to recruit him or others for anything?

2. He was James B. Comey's boss when Comey said Trump was trying to interfere in the FBI's Russia investigation

Former FBI director James B. Comey testified about his interactions with President Trump before the Senate Intelligence Committee June 8. Here are key moments. (Sarah Parnass/The Washington Post)

Sessions is a starring character in the fired FBI chief's testimony. Comey testified last week that Trump shooed Sessions out of the Oval Office so he could be alone with Comey, then asked Comey to back off the FBI's investigation of fired national security adviser Michael Flynn.

“My sense was the attorney general knew he shouldn't be leaving, which is why he was lingering,” Comey told the Senate Intelligence Committee on Thursday.

Comey later told Sessions that he didn't want to be alone with the president. But Comey said he didn't tell Sessions why he was so concerned.

“We considered whether to tell the attorney general, decided that didn't make sense because we believed, rightly, that he was shortly going to recuse,” Comey testified.

Why this is a big deal: A few reasons. Did Sessions suspect that Trump was trying to interfere in the FBI's various investigations of Flynn and Russian meddling? And what did he do about it? Also, why did Comey think he couldn't trust Sessions?

What questions Congress will want to know: Pretty much everything above.

3. Sessions has technically recused himself from the Russia investigation 

Attorney General Jeff Sessions offered to resign his position at one point in recent months, according to two people close to the White House. (Victoria Walker/The Washington Post)

But Comey left open the possibility that Sessions had violated his recusal.

“If, as the president said, I was fired because of the Russia investigation,” Comey testified, “why was the attorney general involved in that chain?”

The Washington Post reported that Trump called Sessions up to the White House to talk about firing Comey, then asked Sessions (and Sessions's No. 2, Deputy Attorney General Rod J. Rosenstein) to explain in writing the case against Comey.

The Post also recently reported that Sessions offered to resign, in part because of Trump's frustration that he stepped aside from the Russia investigation.

Why this is a big deal: Sessions is a Trump ally, and he appears to be caught between the president and his promise to run the Justice Department in an apolitical way. In his confirmation hearing, Sessions promised:

“You simply have to help the president do things that he might desire in a lawful way and have to be able to say no, both for the country, for the legal system and for the president, to avoid situations that are not acceptable.”

What Congress will want to know: Where does Sessions draw the line on recusal? Can he be trusted to not interfere in the department's independent Russia investigation, which is now being led by a special counsel? If it comes down to the Justice Department having to choose whom to believe, special counsel Robert S. Mueller III or the president, whom will Sessions choose?