Here’s a primer on the trial:

The investigation:

Menendez’s relationship with Dr. Salomon Melgen, a close friend and donor, first came under scrutiny in early 2013 after media reports revealed that the New Jersey Democrat was flying on Melgen’s plane to the doctor’s “luxurious” villa in the Dominican Republic. Menendez did not report any of the flights, a potential violation of Senate rules and federal law. When the press disclosed the reports, Menendez cut Melgen a $58,000 check to cover the cost of the flights, but federal prosecutors say there were additional unreported trips.

Menendez intervened on Melgen’s behalf with the Department of Health and Human Services to resolve a $9 million Medicare billing dispute, dragging in then-Senate Majority Leader Harry Reid (D-Nev.), HHS Secretary Kathleen Sebelius, and Center for Medicare & Medicaid Services Administrator Marilyn Tavenner.

Menendez also sought the State Department’s help over a $500 million port security contract Melgen had with the Dominican Republic. Menendez even assisted with visas for several of Melgen’s foreign girlfriends, according to prosecutors.

Melgen and his family members donated hundreds of thousands of dollars in campaign contributions directly to Menendez’s reelection campaign, as well as to a Democratic super PAC that backed Menendez’s 2012 reelection effort.

The charges:

Menendez and Melgen were indicted in April 2015 on a slew of federal corruption charges related to the senator’s alleged acceptance of hundreds of thousands of dollars in improper gifts and campaign contributions as bribes in exchange for using his office to help Melgen. These include eight counts of bribery, three counts of honest services fraud, one count of conspiracy, and one count of violating the Travel Act. Menendez was also charged with one count of making false statements.

Melgen has already been convicted in a multimillion dollar Medicare fraud case in Florida and is awaiting sentencing. Prosecutors may seek as much as 30 years in prison for Melgen in that case, a potential life sentence for the 62-year-old ophthalmologist.

The Justice Department alleges that the Menendez-Melgen relationship turned criminal shortly after the New Jersey Democrat became a senator and went on for nearly seven years.

“The defendants’ bribery scheme began shortly after Menendez’s elevation to the Senate in 2006, when Melgen began a pattern of treating Menendez to weekend and weeklong getaways in the Dominican Republic that would continue for the next several years,” prosecutors said in a recent court filing. “For the first four years of the corruption scheme, the all-expense paid trips Melgen provided often included free roundtrip flights on Melgen’s private jet for Menendez and his various guests. When the doctor’s private jet was unavailable, Melgen supplied equally luxurious travel for the Senator.”

In return for Melgen’s gifts and campaign donations, Menendez reportedly used his Senate office to help the Florida doctor.

“Although Menendez did not pay Melgen back for the lavish gifts in money, he did pay him back using the currency of his Senate office to take official action to benefit the South Florida doctor,” prosecutors wrote. “Email exchanges between the defendants, their agents, and officials from Executive Branch agencies will show Menendez’s considerable efforts to pressure the Executive Branch on Melgen’s behalf. And testimony from the agency officials over whom he exerted that pressure will illuminate the relentlessness of those efforts.”

Menendez is the first senator charged with bribery since the late Sen. Harrison Williams, another New Jersey Democrat, was convicted in 1981 in the Abscam case. The Williams case could come into play again if Menendez is found guilty because of the timing of his ultimate resignation.

The defense:

Menendez has made a variety of arguments to try to rebut the government’s case, and his high-powered defense team — led by Abbe Lowell, a well-respected defense attorney who first came to prominence during former President Bill Clinton’s impeachment trial in 1998 — will use any legal weapon it can.

Menendez has suggested that Cuban intelligence agents were behind a plot to smear him over his opposition to former President Barack Obama’s call to open up U.S. relations with Cuba. There was another version of this defense argument that suggested Obama administration officials wanted to retaliate against Menendez because he opposed them on Cuba and the Iran nuclear deal. Republican donors, as well as supporters of Israel, were sympathetic to this argument, and Menendez raised millions of dollars for his legal defense fund.

In court filings, Menendez’s lawyers argued that the Justice Department violated Menendez’s status under the Speech or Debate Clause, a constitutional prohibition barring lawmakers and aides from facing legal action over legitimate legislative activities. While Menendez failed to get the case thrown out on those grounds, it delayed the trial for two years and will be raised again at points during the trial.

In addition, Menendez will try to challenge the definition of an “official act” based on the Supreme Court decision last year overturning the 2015 conviction of former Republican Gov. Bob McDonnell of Virginia. That ruling made it more difficult to prove corruption cases against politicians in cases where there is no proof of an explicit agreement linking a campaign donation or gift to a vote or official action. U.S. District Judge William H. Walls, who is overseeing the case, hasn’t ruled on a defense motion on this issue yet.

The stakes:

The Menendez trial is expected to last roughly two months. If Menendez is convicted in mid-November, look for a full-court press by Republicans to force him to resign or be expelled immediately. This would allow New Jersey’s GOP governor, Chris Christie, to name his successor — a Republican — before he leaves office in late January. The Republican National Committee is already pointing out that Democrats called for Stevens’ immediate expulsion when he was convicted in 2008.

Senate Democrats would fight that, arguing that Christie shouldn’t get to name Menendez’s replacement since Christie is a lame duck. Democrat Phil Murphy is widely expected to be New Jersey’s next governor, and Murphy would name a Democrat to the post. In a post-Merrick Garland world — where Republicans blocked Obama’s Supreme Court nominee for nearly a year, allowing President Donald Trump to name Neil Gorsuch to the high court — there is less willingness for Democrats to roll over on this issue. It would take a two-thirds majority of the Senate to expel Menendez.

Stevens didn’t leave office until January 2009 after he lost his reelection race that year. And Williams did not resign from the Senate until March 11, 1982, long after his May 1, 1981, conviction.