As the Senate prepares to hold committee hearings on President-elect Donald Trump’s cabinet nominees, Democrats and the top government ethics officer are suggesting that Republicans slow the process down since not all those nominated have completed the necessary ethics review process.

"The announced hearing schedule for several nominees who have not completed the ethics review process is of great concern to me," Office of Government Ethics director Walter Shaub said in a letter to top senators.

The letter was released to the public by Sen.Elizabeth Warren, (D-Mass.), and Senate Minority Leader Chuck Schumer, (D-N.Y.).

"This schedule has created undue pressure on OGE's staff and agency ethics officials to rush through these important reviews," the letter said.

Shaub said he could not recall a time when the Senate held a confirmation hearing without the process being completed.

While there have been only a few nominees who were not confirmed for their positions, the process is designed to be rigorous, and can be contentious. Here’s a look at how it works.

Why does the Senate have to OK a president’s cabinet?

In addition to other statutes and executive orders, Article II, Section 2 of the Constitution says the president "shall nominate, and by and with the advice and consent of the Senate, shall appoint ambassadors, other public ministers and consuls, judges of the Supreme Court and all other officers of the United States, whose appointments are not herein otherwise provided for. ..."

How does the process work?

While the Trump transition team has announced nominees for some positions in the cabinet, those chosen are not officially nominated until Trump takes office on Jan. 20. The announcements are made ahead of the inauguration so candidates can be vetted and the hearing process can begin.

It starts when a presidential transition team nominates a candidate to fill a cabinet position. It is customary for that person to have been vetted, or investigated, by the transition team. It saves embarrassment down the line in the process.

Next, after the Senate is supplied with the nominee’s name, the hearing process begins, usually in the week or two before the inauguration. This year, the hearings begin Tuesday.

The nominations are sent to their relevant committees -- for instance, the Armed Services Committee would hold hearings on the secretary of defense nominee -- for a hearing to be scheduled.

After the hearings, in which senators question the nominee on virtually anything they wish, one of two things will happen: The nomination will be sent to the floor for a full Senate vote with a favorable recommendation, an unfavorable recommendation or no recommendation at all; or the nomination will not be sent to the Senate floor.

If the nomination is not sent to the floor of the Senate, it doesn’t mean that the nomination is dead. Senators can invoke cloture to move the nomination to the floor for a vote. Cloture is the only procedure by which the Senate can vote to set an end to a debate. Cloture sets a time limit on debate over a nominee. The time limit is 30 hours.

Cloture keeps opponents of the nominee from endlessly debating the qualifications of a candidate, keeping a nomination from reaching the Senate floor for a vote and keeping the position for which the person nominated open.

But a rule that grew out of a Democratic-led measure in 2013 led to what is now called the “nuclear option,” which means that with 51 votes, instead of 60 votes, filibusters against nominees are basically prohibited, except for Supreme Court nominees. So while a vote can be delayed, it cannot be killed and will eventually come to the floor.

After cloture is called, the Senate must wait until the second calendar day after to hold a vote to end debate. The debate can be ended on a simple majority vote with 51 votes. Before 2013, it took 60 votes, but a Democratic-led Senate voted that year to change the rules of the Senate and require only 51 votes to end debate and vote.

Once a nomination moves to the Senate floor for a vote, it takes a simple majority there as well to confirm the nomination. There are currently 52 Republican senators and 48 Democratic senators.

When are they confirmed?

While the hearings will begin Tuesday, no nominee is confirmed before Trump takes office on Jan. 20. Hearings begin before presidents take office to minimize the time between the inauguration and when the new administration as a whole can get to work.

I’m hearing that not all the nominees have been vetted yet. What does that mean?

The practice of vetting – or confirming the information provided by the nominee – generally happens before the person’s name is made public as the president’s choice for a position. Usually, the Federal Bureau of Investigation conducts a background investigation, in addition to having his or her financial information certified by ethics officers.

Nominees fill out questionnaires, such as the White House’s “Personal Data Statement Questionnaire,” and the Office of Government Ethics' Standard Form 278. They answer about 230 questions about their mental health, their in-laws, any drug convictions, whether they hired maids or gardeners and their personal alcohol use. The questions are designed to head off any embarrassing information coming out at Senate hearings.

What happens if a nominee isn’t approved?

It doesn’t happen often, but when it does, the process begins again. Trump must nominate someone else.

How many potential cabinet members must be confirmed by the Senate?

All cabinet-level officials must have Senate approval, along with more than 1,000 other senior positions and agency heads.

Chief of staff and other advisory positions in the White House, such as national security adviser, do not require Senate approval.

Here’s a list of the Senate committee hearings scheduled so far:

• Attorney general: Jeff Sessions, 9:30 a.m. Tuesday and Wednesday

• CIA director: Mike Pompeo, 10 a.m. Wednesday

• Director of homeland security: John Kelly, 2 p.m. Wednesday

• Education: Betsy DeVos, 10 a.m. Wednesday

• Labor: Andy Puzder, Jan. 17 (tentative)

• Secretary of state: Rex Tillerson, Wednesday and Thursday (tentative)

• Transportation: Elaine Chao, 10:15 a.m. Wednesday

• UN ambassador: Nikki Haley, Jan. 18 (tentative)

• Housing: Ben Carson, 10 a.m. Thursday