The jury trying to decide Sen. Bob Menendez’s (D-NJ) corruption case is deadlocked. The foreman sent word to US District Court Judge William Walls on Monday that they could not reach unanimous agreement on any of the charges Menendez is facing.

The trial’s not over yet — Judge Walls dismissed the jury and requested that they continue deliberations again the next day. But the chances that it will end in a mistrial seem to be rising.

Prosecutors are alleging that Menendez accepted expensive gifts from a wealthy supporter and friend, Dr. Salomon Melgen, in exchange for intervening on behalf of the doctor’s business interests with government officials (and trying to get visas for the doctor’s girlfriends). Menendez asserts he’s done nothing wrong.

The case has been closely watched nationally because the outcome could have major implications for the balance of power in the United States Senate and for the Republican agenda. Though a conviction can’t by itself remove him from office, if there is one, it would put pressure on Menendez to resign from the Senate. GOP leaders could then hold a vote on whether he should be expelled — a vote that would need two-thirds Senate support to succeed.

Democrats probably wouldn’t care all that much if Menendez was driven to resign or expelled — so long as it happens after mid-January. That’s because New Jersey’s governor will still be a Republican, Chris Christie, until then. Christie would be sure to replace Menendez with a Republican — something that would increase the GOP majority to 53 seats and perhaps even give the party another shot at Obamacare repeal.

But Democrat Phil Murphy won November’s New Jersey governor’s election, so if there’s a change after that, he’ll simply replace Menendez with another Democrat. And if convicted before then, Menendez could conceivably announce he’d resign effective in January — giving enough Democrats an excuse not to vote to expel him.

As things are, though, it seems increasingly likely that things will end in a mistrial. After that, the government could theoretically charge Menendez again — but the partisan stakes of replacing him would be much lower with Murphy in office.

1) Who is Bob Menendez, and what is he on trial for?

Menendez, the son of Cuban immigrants, rose to political prominence in Hudson County, New Jersey, in the 1980s, before winning a seat in the House of Representatives in 1992 and then an appointment to a US Senate vacancy in 2006. He’s a former chair of the Democratic Senatorial Campaign Committee and of the Senate Foreign Relations Committee. His politics are standard-issue liberal on most matters, though he’s notably more hawkish than most Democrats on foreign policy topics such as Cuba and Iran.

But Menendez fell under Justice Department scrutiny for his relationship with a wealthy Florida eye doctor, Salomon Melgen, whom he often traveled with. In April 2015, prosecutors filed corruption charges against Menendez (and against Melgen), arguing that Menendez used his position to benefit Melgen in return for items of value.

Prosecutors alleged that Melgen gave Menendez:

Many flights on his company’s private jets, and at least one first-class commercial flight (Menendez didn’t initially disclose these in Senate ethics filings, and paid Melgen back for some of the flights years later)

Use of a Caribbean villa, access to a Dominican resort, and a stay at a luxury Paris hotel

Tens of thousands of dollars for his legal defense fund

Hundreds of thousands of dollars earmarked to be spent to help his 2012 reelection effort

In return, they allege, Menendez did several things for Melgen:

He met with then-Health and Human Services Secretary Kathleen Sebelius over a billing dispute Melgen was having with Medicare, and advocated on Melgen’s behalf. (In a separate trial, Melgen was convicted earlier this year of defrauding Medicare of more than $90 million from charges stemming from this dispute.)

He tried to get the State Department to push the Dominican Republic to uphold a contract Melgen had to provide cargo screening in the country’s ports.

He tried to stop US Customs and Border Protection from donating cargo screening equipment to the Dominican Republic, because of that same port contract Melgen had.

He had his staffers help get tourist or student visas for three of Melgen’s foreign girlfriends.

Menendez has maintained his innocence, and said that while Melgen was a close friend, he did nothing illegal on the doctor’s behalf.

2) Which issues are most important for the trial’s outcome?

In attempting to prove public corruption charges against a politician, prosecutors have to make the case that the defendant knowingly abused his power as part of a corrupt quid pro quo.

That is: these prosecutors have to prove that Menendez did or promised to do “official acts” that Melgen wanted, and that these acts were done in return for the various gifts and other benefits that Melgen sent his way.

There’s a moderately high bar for just what counts as an “official act.” In a ruling last year, the Supreme Court overturned former Virginia Gov. Bob McDonnell’s corruption convictions, because they related mainly to meetings he had set up and events he had hosted on behalf of a donor.

"Setting up a meeting, talking to another official, or organizing an event — without more — does not fit that definition of ‘official act,’" the justices wrote. Instead, they said, such an act must involve “a formal exercise of government power.” An actual policy change would count, and the justices also said that exerting “pressure” on another official to change policy or even simply promising to change policy would also count.

Prosecutors have argued that Menendez tried to pressure HHS, State, and Customs officials on Melgen’s behalf in specific meetings and interactions. However, they have presented no evidence that Menendez did those things for Melgen specifically in return for hotel stays, flights, and donations. As a result, the defense tried to get the bribery charges thrown out.

But Judge Walls agreed with prosecutors that the case could go forward, saying essentially that Melgen could have been paying off Menendez to get his official help at a later date, even if he had no specific demands at the time. "Payment may be made with intent to retain official services on an as-needed basis,” Walls said.

3) What happens to Menendez’s seat if he is convicted?

Procedurally, nothing would happen. “Members of Congress do not automatically forfeit their offices upon conviction of a crime that constitutes a felony,” Jack Maskell of the Congressional Research Service writes. “No express constitutional disability or “disqualification” from Congress exists for the conviction of a crime, other than under the Fourteenth Amendment for certain treasonous conduct.”

If Menendez were to serve a prison sentence, he of course wouldn’t be able to show up for votes in the Senate — but even that wouldn’t automatically divest him of his seat. In any case, it’s highly unlikely to happen anytime soon, because Menendez would appeal any conviction and accompanying sentence, and that would be a lengthy process.

Instead, the conversation about a post-conviction Menendez’s seat would focus on two possibilities: resignation or expulsion.

Past precedent indicates that resignation has been the norm. Only four sitting senators have ever been convicted of crimes, and three resigned after they were convicted. (The fourth held on to his seat and appealed his conviction but then happened to die a few months later.)

If Menendez chooses not to resign after a conviction, though, there’s only one way he can be involuntarily deprived of his seat: Two-thirds of the Senate would have to vote to expel him from the chamber.

A conviction would surely lead to a major expulsion push from Republican senators. They would argue that allowing a senator convicted of federal corruption charges to hold on to his seat is outrageous. And many of them would also salivate at the prospect of gaining Menendez’s seat for their own party through an appointment from Christie.

4) What’s the history of expulsion from the Senate?

There’s very little precedent for it. According to the Senate Historical Office, the Senate has only ever expelled 15 members. That seems like a lot, but 14 of those were expelled for supporting the Confederacy during the Civil War. The other instance occurred all the way back in 1797, when Sen. William Blount of Tennessee was expelled for treason.

Now, there have been a few unsuccessful expulsion votes since the Civil War. Sen. Reed Smoot (R-UT) was targeted for expulsion because he was a Mormon, and he survived. Sen. Bob La Follette (R-WI) was targeted for expulsion for arguing against US entry into World War I, but he survived too. A few senators also survived expulsion votes motivated by corruption scandals, most recently Sen. William Langer (R-ND) in 1942.

Still, one of the reasons we haven’t seen successful expulsion votes is because senators have resigned rather than face an expulsion vote they expected to lose. The most recent senator to do so was Bob Packwood (R-OR) in 1995. When the Senate Ethics Committee recommended his expulsion over a sexual misconduct and abuse of power scandal, he resigned.

Finally, there’s another historical precedent that Menendez is probably well aware of — that of Sen. Harrison Williams (D-NJ), who held the very same Senate seat Menendez now occupies.

Williams was convicted for bribery in the Abscam scandal in May 1981 — but he didn’t resign his seat right away, and continued to maintain his innocence. The Senate Ethics Committee looked into the matter and unanimously recommended his expulsion in August 1981.

Yet after that, it took several more months before the Senate — Republican-controlled, but in an era with less partisan polarization — moved toward voting on the expulsion resolution. Once debate did begin and it become clear to Williams that he’d lose the vote, he resigned to avoid the embarrassment in March 1982.

5) What are the prospects for an expulsion vote?

Republicans’ chances to wrest Menendez’s seat from Democrats’ hands have lately dropped, but if Menendez is convicted of even one charge, they may well make an attempt to do so — or at least to embarrass Senate Democrats — by holding an expulsion vote.

To have a chance at flipping the seat, they’d have to act quickly. Phil Murphy will be sworn in as New Jersey’s governor on January 16, 2018 — so a successful expulsion would have to take place sometime before then, while Chris Christie is still in office to appoint a Republican as Menendez’s replacement.

And even though the threshold for success on an expulsion vote is high (two-thirds of the Senate), holding a quick vote would be a win-win for Republicans. Either Menendez is in fact expelled, and the GOP gets his seat, or at least 34 Democrats would have to cast a politically toxic vote to let a felon convicted of corruption keep his seat.

Now, if this were to happen after Murphy was sworn in, expelling a senator convicted of corruption charges could well be a matter of bipartisan agreement — though it depends what he’d actually be convicted of doing.

But this year, Democrats are deathly afraid of handing the GOP another seat. So until mid-January, enough of them will likely find a way to block any expulsion of Menendez if he is convicted of anything.