New Jersey Sen. Menendez bribery trial begins, with possible implications for Washington

— -- A defiant Sen. Robert Menendez arrived at a federal courthouse in Newark, New Jersey Wednesday morning for the start of a long-awaited federal corruption trial.

Menendez, D-N.J., is accused of accepting nearly $1 million in gifts and campaign donations from his friend, Florida doctor Salomon Melgen, in exchange for political and personal favors.

According to the indictment, among the favors provided using his political office, Menendez secured visas for several of Melgen’s girlfriends and advocated on Melgen’s behalf for a Medicaid billing dispute that was worth approximately $8.9 million to the doctor.

Menendez accepted flights aboard Melgen’s private jets, vacations at Melgen’s private villa in the Dominican Republic, and three nights at a luxury hotel in Paris to spend a weekend with his girlfriend, the indictment alleges. Melgen also gave Menendez $750,000 in campaign donations, according to the Justice Department. None of the gifts Menendez accepted were disclosed, the DOJ says.

At the time the indictment was issued in 2015, then-Assistant Attorney General for the Criminal Division Leslie R. Caldwell portrayed the matter as a potential act of "government corruption." The senator had reportedly been under investigation by a grand jury as early as 2013.

Menendez has maintained his innocence, saying Wednesday as he arrived at the federal courthouse in Newark: "Never, not once, not once, have I dishonored my public office."

Political Implications

Along with the legal ramifications for Menendez, the case carries potentially enormous political consequences for the balance of power in the United States Senate.

Republicans hold a slim 52-48 majority in the Senate, but the recent legislative battle over healthcare laid bare the divisions within the Republican Party this summer.

If Menendez is convicted, he will face mounting pressure to resign his seat, which would then allow New Jersey Governor Chris Christie – a Republican whose term ends in January 2018 – to appoint his replacement until the seat is filled via election.

An additional Republican vote in the Senate could carry huge implications for the passage of bills on any number of hot button issues, such as healthcare, tax reform and immigration.

Menendez has stood firm in saying this will not happen, and congressional Democrats have largely stood by him. But if the trial results in a conviction, the political pressure will be intense for Democrats to break with Menendez.

If he is convicted and refuses to resign, Menendez could be removed from the Senate, if two-thirds of his colleagues vote for his removal. This has not happened since the 1860s, when numerous senators were removed for supporting the Confederacy.

Further complicating the process is the fact that 2017 is an election year in New Jersey, and Christie is largely expected to be replaced by Democrat Phil Murphy, who has been leading in the polls over Republican Lieutenant Governor Kim Guadagno.

The laws governing how special elections are held in New Jersey largely leave that power at the discretion of the governor.

The law states that if a vacancy occurs, the election for a permanent replacement will happen at the next general election in the state, unless it occurs at least 70 days before the next primary election in the state. However, the law also states that these are the rules “unless the governor shall deem it advisable to call a special election.”

This means that an election to permanently replace Menendez could wait as late as November 2018, but could really come whenever the sitting governor decides -- be that Christie or his replacement.

Despite the intrigue surrounding his trial, Menendez is still fundraising and campaigning for re-election in 2018, and was defiant when arriving in federal court this morning, telling reporters, “I believe when all of the facts are known, I will be vindicated.”

For the time being, Menendez indicated that he is willing to miss days in court in order to be present for votes in the Senate, depending on the role his presence in Washington could play.

"The Constitution, like any citizen, gives me the right to ultimately assist in my defense in court, and I intend to be here every day. The Constitution also gives me the right to cast a vote on behalf of the people I represent in the United States Senate," said Menendez Wednesday. "When the conflict exists, if it becomes a conflict, a clash between those constitutional rights, I will make a decision based upon the gravity of the situation and difference that my vote could make."

ABC News' Aaron Katersky contributed to this report.