The Democratic field of presidential candidates could narrow again this week, with the Iowa caucuses — the first measurable test of the primary season — a month away.

New Jersey Sen. Cory Booker ended his bid for the Democratic nomination on Jan. 13. Texan Julian Castro, a former U.S. housing secretary and mayor of San Antonio, dropped out on Jan. 2.

"To my team, supporters, and everyone who gave me a shot—thank you. I am so proud of what we built, and I feel nothing but faith in what we can accomplish together," Booker said.

The Democratic primary season begins in earnest on Feb. 3 with the Iowa caucuses.

Booker was preceded to the exits by Texan Julian Castro California Sen. Kamala Harris, state governors Steve Bullock, John Hickenlooper and Jay Inslee, sitting congressmen Tim Ryan, Eric Swalwell and Seth Moulton, New York City Mayor Bill de Blasio, Sen. Kirsten Gillibrand and former Beto O'Rourke.

The race in November saw the emergence of late entrants – former Massachusetts governor Deval Patrick and ex-New York City Mayor Michael Bloomberg. Both men had initially sworn off running for the nomination but say they were compelled to change their minds due to a lack of a clear front-runner.

Some lesser-known politicians who have signalled their improbable bids for president are still technically in the race, including former U.S. congressman John Delaney from Maryland and self-help author Marianne Williamson.

Here are the remaining original contenders. Read all of the profiles or click on a name to go straight to their profile:

RUNNING

(Yuri Gripas/Reuters)

On May 2, Michael Bennet became the 21st candidate for the Democrats, and the umpteenth one with some kind of law degree. He has served Colorado as senator since 2009, and is making his bid despite a recent prostate cancer diagnosis (he had surgery and said he's now cancer-free).

Before becoming a U.S. senator, Bennet had a turn in the State Department under Bill Clinton and held private- and public-sector jobs in Ohio and Colorado, including as superintendent of the Denver public school system. It will be fascinating to see Bennet tussle with fellow candidate John Hickenlooper on a debate podium — Bennet served as chief of staff in 2003-2004 to Hickenlooper during the latter's stint as governor of Colorado.

Pub trivia: Bennet gained media attention this past January for his heated response on the Senate floor to Republican Ted Cruz over the government shutdown. Bennet has one thing in common with the Calgary-born Cruz: He is aiming to be the first president to fall under the natural-born citizen clause of the Constitution, which allows for foreign-born candidates in certain cases. Bennet was born in India, the result of his father's career as a U.S. diplomat.

(Manuel Balce Ceneta/The Associated Press)

Biden has a compelling story of overcoming personal loss, and brings tons of experience to bear — he was a senator for more than three decades and a two-term vice-president under Barack Obama. Biden lives in Delaware, but was raised in Scranton, Penn., and his working-class charm could help swing much-needed Pennsylvania back to the Democratic column. On the other hand, he has been prone to verbal gaffes, and ran two unsuccessful presidential campaigns (1988, 2008).

His touchy-feely ways have recently come under renewed scrutiny in the #MeToo era, as well as his prominent role in grilling Anita Hill, who accused future Supreme Court Justice Clarence Thomas of sexual harassment in 1991 hearings.

Biden, after much speculation, released a video on April 25 announcing his intention to run.

Pub trivia: Biden would be days shy of 78 on election day. If he wins, it would make him by far the oldest president to take office.

(Win McNamee/Getty Images)

The self-described democratic socialist won over 20 states and territories during the 2016 Democratic primaries, which is why he can't be overlooked. As well, the second contest is in New Hampshire, adjacent to Sanders' home state of Vermont, where he won by 22 percentage points over Hillary Clinton. But it's questionable whether Sanders would do as well in a crowded field as in a race when he was the only serious threat — sorry, Martin O'Malley — to a very establishment candidate in Clinton. As well, Sanders turns 78 in 2019, which would make him 87 and change by the end of a two-term presidency.

Pub trivia: In addition to his Jewish heritage, a Sanders presidency would be noteworthy because he is an independent (although he caucuses with the Democrats).

(Stephen Maturen/Getty Images)

If Klobuchar were an athlete, sportswriters would trot out "she just wins" clichés to explain why she's a strong contender. The Minnesotan just came off the most competitive of her four U.S. Senate elections in November — and her margin of victory was 24 percentage points.

A lawyer, she also won two elections for district attorney in her county and bet on the right pony in the last two contested Democratic presidential primaries — backing Obama in 2008 and Hillary Clinton in 2016. She has co-sponsored bills with Republicans in the Senate and will be perceived as one of the centrist candidates in the Democratic race.

Her composure was on display during last year's contentious Supreme Court nomination hearings, when Brett Kavanaugh apologized for lashing out at her when she asked if he had a drinking problem in the past.

Klobuchar, who announced her bid on Feb. 10, would look to avoid the same fate of Minnesotans in modern presidential elections: Hubert Humphrey (1968) lost closely, Walter Mondale (1984) did not.

Pub trivia: If nominated or elected, she would overtake Melania Trump and others as the most well-known Slovenian-American. She was also a published author by 22, writing a book, Uncovering the Dome, about the controversial process that saw taxpayers foot much of the bill for the Hubert H. Humphrey Metrodome for the Vikings and Twins in the 1980s.

(Brian Snyder/Reuters)

From appearances as a personal finance expert on Dr. Phil beginning in 2003 to her subsequent U.S. Senate career, Warren's emphasis on pocketbook issues and consumer protections could resonate with voters tired of so-called identity politics. Warren and Trump have traded verbal jabs – she has attacked him for being a "wannabe tyrant" overseeing a corrupt administration, while he's mocked her claims to Indigenous ancestry. Warren's profile as a northeastern liberal might, on the surface, seem a challenge in winning over voters in the U.S. heartland, but Warren could play up the fact she was raised in a churchgoing Methodist family in Oklahoma.

Warren officially launched her campaign in the working-class city of Lawrence, Mass., on Feb. 9.

Pub trivia: Warren, married to her current husband (Bruce Mann) since 1980, retains the surname of a previous spouse.

(City of South Bend, Indiana/Handout/Reuters)

​Buttigieg (pronounced BOO-ti-jij), 37, is the mayor of South Bend, Ind., and a veteran who served in Afghanistan. The Harvard grad and Rhodes Scholar announced Jan. 23 that he has launched an exploratory committee in a video he posted to Twitter. The relative unknown says he represents a new generation of leadership with fresh approaches to the country's problems.

"When it comes to experience right now, nothing could be more relevant than leading one of America's turnaround cities," he told reporters.

He did manage to get on the radar of at least one prominent politician. Former president Barack Obama mentioned Buttigieg in an interview shortly before leaving the White House as a young Democrat with a bright future.

Pub trivia: If he wins the party's nomination, he would become the first openly gay nominee of a major U.S. political party.

FILE - In this Tuesday, April 11, 2017 file photo, Democratic Rep. Tulsi Gabbard speaks at the Kona Town Hall Meeting at Kealakehe Intermediate School in Kona, Hawaii. She is running against Republican Brian Evans for Hawaii's second congressional seat. (, File) (Laura Ruminski/West Hawaii Today via AP)

Gabbard, 37, told CNN in an interview that aired on Jan. 12 that she has decided to run for president, and she made it official on Jan. 24.

On Election Night 2012, MSNBC anchor Rachel Maddow hailed incoming Hawaii congresswoman Gabbard as being "on the fast track to being very famous some day." She was a rare commodity for Democrats in the House, a female military veteran who served in Iraq (2004-2006). Gabbard can't be accused of trying to win popularity contests within her party. She endorsed Bernie Sanders over Hillary Clinton in the 2016 campaign, criticized America's "endless wars" overseas and controversially met with Bashar al-Assad while on a Middle East trip in early 2017, not advising Democratic leaders beforehand.

While her salty put-down of the Trump administration for its response to the killing of U.S. citizen Jamal Khashoggi was widely noted, previous comments Gabbard has made on same-sex marriage rights and Islam have invited criticism from progressive party members.

Pub trivia: Gabbard would be the youngest president at inauguration, the first modern president not born in the 50 states — having been born in the U.S. territory American Samoa — and the first Hindu-practising president.

(Scott Morgan/Reuters)

Yang, who was raised and schooled in the Northeast, launched his campaign in Nov. 2017, receiving little notice from the mainstream media given his lack of a high-profile political resumé. But the savvy longtime tech entrepreneur has marshaled much support online, converting enthusiasm from the so-called "Yang Gang" into the requisite amount of donations to keep qualifying for debates. His platform mixes the humanistic — a universal income "Freedom Dividend" plan for all working-age Americans — with cold-water tech reality, as he believes untold millions could lose jobs in the coming years due to automation and that the country and political class is woefully unprepared for that possibility.

Pub trivia: Of Taiwanese descent, he would be the first Asian-American president. He also wants to make good on a promise he made, one which would threaten to break the internet: Campaigning in both human and hologram form.

Out, then in

Tom Steyer , billionaire long in favour of Trump impeachment, reconsidered on July 9, 2019.

, billionaire long in favour of Trump impeachment, reconsidered on July 9, 2019. Deval Patrick , former Massachusetts governor, changed mind from original stance on Nov. 14, 2019.

, former Massachusetts governor, changed mind from original stance on Nov. 14, 2019. Former New York City mayor Michael Bloomberg, back in on Nov. 25, 2019.

In, then out

California congressman Eric Swalwell , ended campaign on July 8, 2019.

, ended campaign on July 8, 2019. Former Colorado governor John Hickenlooper packed it in on Aug. 15, 2019.

packed it in on Aug. 15, 2019. Washington State Governor Jay Inslee said he was withdrawing Aug. 21, 2019.

said he was withdrawing Aug. 21, 2019. Rep. Seth Moulton of Massachusetts, out on Aug. 23, 2019.

of Massachusetts, out on Aug. 23, 2019. Sen. Kirsten Gillibrand , out on Aug. 28, 2019.

, out on Aug. 28, 2019. NYC Mayor Bill de Blasio , stepped aside on Sept. 20, 2019.

, stepped aside on Sept. 20, 2019. Ohio Rep. Tim Ryan , leaves race on Oct. 24, 2019.

, leaves race on Oct. 24, 2019. Former Texas congressman Beto O'Rourke , ends campaign on Nov. 1, 2019.

, ends campaign on Nov. 1, 2019. Small-town Florida mayor Wayne Messam suspends campaign on Nov. 20, 2019.

suspends campaign on Nov. 20, 2019. Montana governor Steve Bullock ended his campaign Dec. 2.

ended his campaign Dec. 2. Sen. Kamala Harris of California dropped out of the race Dec. 3.

Definitely not running