Sen. Cory Booker’s (D-N.J.) decision to testify Wednesday against Sen. Jeff Sessions Jefferson (Jeff) Beauregard SessionsTrump's policies on refugees are as simple as ABCs Ocasio-Cortez, Velázquez call for convention to decide Puerto Rico status White House officials voted by show of hands on 2018 family separations: report MORE (R-Ala.), President-elect Donald Trump Donald John TrumpBarr criticizes DOJ in speech declaring all agency power 'is invested in the attorney general' Military leaders asked about using heat ray on protesters outside White House: report Powell warns failure to reach COVID-19 deal could 'scar and damage' economy MORE’s pick for attorney general, is the first clear signal that the 2020 maneuvering in the upper chamber has begun.

Booker is taking the unprecedented step of testifying against a fellow senator who has been selected for a Cabinet post.

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The 47-year-old is one of a handful of Senate Democrats viewed as promising candidates for the White House in 2020 at a time when few governors are considered top-flight options to challenge Trump in four years.

He is using a strategy that other senators are expected to employ over the next several years to boost their national profile: taking a hard line against the Trump administration.

The others are Sen. Elizabeth Warren Elizabeth WarrenNo new taxes for the ultra rich — fix bad tax policy instead Democrats back away from quick reversal of Trump tax cuts It's time for newspapers to stop endorsing presidential candidates MORE (Mass.), who has a large loyal following within the liberal grass roots; Sen. Kirsten Gillibrand Kirsten GillibrandSuburban moms are going to decide the 2020 election Jon Stewart urges Congress to help veterans exposed to burn pits The Hill's Campaign Report: 19 years since 9/11 | Dem rival to Marjorie Taylor Greene drops out | Collin Peterson faces fight of his career | Court delivers blow to ex-felon voting rights in Florida MORE (N.Y.) and Sen. Amy Klobuchar Amy KlobucharEPA delivers win for ethanol industry angered by waivers to refiners It's time for newspapers to stop endorsing presidential candidates Biden marks anniversary of the Violence Against Women Act, knocks Trump and McConnell MORE (Minn.), two leading women in the upper chamber; Sen. Chris Murphy Christopher (Chris) Scott MurphyGOP chairman to release interim report on Biden probe 'in about a week' This week: House returns for pre-election sprint Battle over timing complicates Democratic shutdown strategy MORE (Conn.), who has made a name for himself on gun control; Sen. Sherrod Brown Sherrod Campbell BrownEmboldened Democrats haggle over 2021 agenda Hillicon Valley: Russia 'amplifying' concerns around mail-in voting to undermine election | Facebook and Twitter take steps to limit Trump remarks on voting | Facebook to block political ads ahead of election Top Democrats press Trump to sanction Russian individuals over 2020 election interference efforts MORE (Ohio), who represents a pivotal battleground state; and Sen. Tim Kaine Timothy (Tim) Michael KaineThe Hill's Morning Report - Sponsored by The Air Line Pilots Association - Pence lauds Harris as 'experienced debater'; Trump, Biden diverge over debate prep Catholic group launches .7M campaign against Biden targeting swing-state voters GOP senator to quarantine after coronavirus exposure MORE (Va.), Hillary Clinton Hillary Diane Rodham ClintonBarr criticizes DOJ in speech declaring all agency power 'is invested in the attorney general' Virginia Democrat blasts Trump's 'appalling' remark about COVID-19 deaths in 'blue states' The Hill's Campaign Report: Biden asks if public can trust vaccine from Trump ahead of Election Day | Oklahoma health officials raised red flags before Trump rally MORE’s running mate in 2016.

Warren has been the most active of this group in rallying her demoralized party to stand up to the president-elect, focusing her criticism on Sessions and senior Trump adviser Stephen Bannon.

Senate Democratic Leader Charles Schumer Chuck SchumerDemocrats scramble on COVID-19 relief amid division, Trump surprise Pelosi, Schumer 'encouraged' by Trump call for bigger coronavirus relief package Schumer, Sanders call for Senate panel to address election security MORE (N.Y.) struck a conciliatory tone right after the election by pledging to look for common ground with Trump, though he has also vowed to fight the incoming administration when necessary. The Democratic base wants the party to take on Trump, whom many liberals view with fear and disdain.

Booker’s plan to appear before the Judiciary Committee on Wednesday in an attempt to derail Sessions’s confirmation is being viewed as a bold move to raise his profile.

“It takes a lot of guts for a United States senator to go in to offer testimony in opposition to one of his colleagues. … I can’t remember this ever happening,” said Tad Devine, a Democratic strategist who served as a senior adviser to Sen. Bernie Sanders Bernie SandersSenate Republicans signal openness to working with Biden Hillicon Valley: DOJ indicts Chinese, Malaysian hackers accused of targeting over 100 organizations | GOP senators raise concerns over Oracle-TikTok deal | QAnon awareness jumps in new poll Schumer, Sanders call for Senate panel to address election security MORE’s (I-Vt.) presidential campaign last year.

The testimony will give Booker, who is black, plenty of national media attention and a platform to address civil rights, which Democratic groups have flagged as a major concern with respect to Sessions being attorney general.

“He is someone with a demonstrated leadership in civil rights and he feels very strongly about Sen. Sessions’s civil rights record,” said Steve Elmendorf, a Democratic lobbyist and strategist. “We’re in a unique moment here where we have a new president who’s trying to put a bunch of people into office who are extreme.”

“In the face of the extremism, Democrats are going to stand up and say something,” he added.

Republicans, however, say Booker is breaching Senate traditions of comity for the sake of his own his presidential ambitions.

“I’m very disappointed that Sen. Booker has chosen to start his 2020 presidential campaign by testifying against Sen. Sessions,” Sen. Tom Cotton Tom Bryant CottonLoeffler calls for hearing in wake of Netflix's 'Cuties' Health care in the crosshairs with new Trump Supreme Court list Cruz says he wouldn't accept Supreme Court nomination MORE (R-Ark.) said in a statement posted on Facebook.

He slammed Booker’s testimony as “a disgraceful breach of custom” and questioned his motives by noting that last year the New Jersey legislator said he was “honored” to partner with Sessions on a resolution honoring civil rights activists.

A spokesman for Booker pointed to his boss’s statement from earlier this week: “I do not take lightly the decision to testify against a Senate colleague. … Senator Sessions’ decades-long record is concerning in a number of ways, from his opposition to bipartisan criminal justice reform to his views on bipartisan drug policy reform, from his efforts earlier in his career to deny citizens voting rights to his criticism of the Voting Rights Act, from his failure to defend the civil rights of women, minorities, and LGBT Americans to his opposition to common sense, bipartisan immigration reform.”

Booker is up for reelection in 2020. New Jersey state officials could not be reached for comment at press time on the question of whether Booker can run for the Senate and the White House simultaneously. A Nov. 14 report on NJ.com suggests that Booker would have to choose one.

While Democrats with possible White House ambitions are figuring out how to best position themselves for 2020, it’s a topic they don’t want to discuss publicly.

“I’m not sure that anybody running for president in 2020 is going to tell you in January of 2017,” Murphy told The Hill.

When asked about her interest in running for president, Klobuchar on Tuesday quickly ducked into the Senate Democratic lunch, telling a reporter, “I’ve got to eat food.”

Later she said, “I’m representing Minnesota in the Senate, and we haven’t even sworn in this president.”

“I’m not thinking about that at all,” said Klobuchar, who recently decided against a run for governor.

Elmendorf and other Democratic strategists say the Senate bench of potential presidential candidates is deep, naming Booker, Warren, Klobuchar, Gillibrand, Murphy, Kaine and Brown.

Kaine, however, has said publicly he won’t run in 2020.

Others on the list include Colorado Sen. Michael Bennet Michael Farrand BennetOVERNIGHT ENERGY: House Democrats tee up vote on climate-focused energy bill next week | EPA reappoints controversial leader to air quality advisory committee | Coronavirus creates delay in Pentagon research for alternative to 'forever chemicals' Senate Democrats demand White House fire controversial head of public lands agency Next crisis, keep people working and give them raises MORE and Missouri Sen. Claire McCaskill Claire Conner McCaskillDemocratic-linked group runs ads in Kansas GOP Senate primary Trump mocked for low attendance at rally Missouri county issues travel advisory for Lake of the Ozarks after Memorial Day parties MORE.

“There are a lot,” Elmendorf said. “I am a big believer that we need younger people, new faces and all of those people as new faces.”

The biggest challenge of these senators — with the exception of Warren, who has a prominent national profile — is the need to raise their name identification with voters and donors outside the Beltway.

“Donald Trump is going to give people a great opportunity to build that profile. If you’re a Democrat, you have a lot of opportunities to show you can stand up to this guy, which people in the party want to see,” Elmendorf added.

A potential complication for senators viewing possible White House runs four years from now is that many of them face reelection in 2018.

Brown, Gillibrand, Klobuchar, McCaskill, Murphy and Warren have to win reelection next year. Brown and McCaskill are running in states that Trump won by healthy margins and may have tough races.

Brown on Tuesday said he is running for a full Senate term in 2018 and is not interested in vying for the Democratic presidential nomination two years later.

“I have no interest. I want to do this,” he told The Hill, referring to his Senate job.

McCaskill said she also plans to serve a full six-year term if reelected next year.

“Yeah, I don’t think that’s a problem,” she said. “I don’t think that’s an issue at all. I’d be more worried about a problem with my family’s health than I would be running for president.”

Democratic strategists, however, say running for Senate reelection in 2018 could serve as a springboard for senators to White House campaigns in 2020 by giving them the chance to reach out to donors around the country.

“George W. Bush won a very impressive election for governor of Texas in 1998 and that very much helped to launch his presidential campaign in 2000,” Devine said.

“Hillary Clinton won reelection in 2006 [to the Senate before the 2008 presidential campaign] and raised a lot of money,” he added. “It really depends on how you do in the campaign.

“Can you win a decisive victory that can demonstrate to people outside your state that you have the capacity to win voters from the other party and independents? Can you raise a lot of resources, not just from within your own state but from around the country?”