Among Senate Democrats’ broad opposition to President Donald Trump’s Cabinet, six stand out in having voted against nearly all of his nominees — and each senator just happens to be a potential Trump challenger in 2020.

The Democrats’ latest hell-no caucus includes Kirsten Gillibrand of New York, Kamala Harris of California, Elizabeth Warren of Massachusetts, Cory Booker of New Jersey, Jeff Merkley of Oregon and Vermont independent Bernie Sanders, according to a POLITICO analysis of votes on 35 Trump nominees confirmed this year.

With the Senate having taken up virtually no major legislation as it nears its sixth month in session, these votes offer the best clue to who are positioning themselves as leaders of a party eager to take the fight to Trump.

Among Senate Democrats, Gillibrand has taken the most aggressive stance toward Trump's nominees, only voting for only a single one — though other potential 2020 contenders weren't far behind.

The hell-no Democrats brush off suggestions that future political prospects play into their decision to resist even lower-level Trump picks, including Rod Rosenstein as the second-ranking official at the Justice Department and former Iowa Gov. Terry Branstad as U.S. ambassador to China.

But all six senators are often on the party’s short lists to challenge Trump in 2020, and they’ve all cemented their following among the liberal grass roots by mounting such a fierce opposition to his nominees.

“Lost in all of the obvious concern about Russia is the fact that Trump is pushing an extremely, extremely right-wing, reactionary agenda: tax breaks for billionaires, throwing 24 million people off health insurance, and massive cuts to programs that working people need,” Sanders said in a brief interview. “And many of his appointees are pushing exactly that agenda, and I’m not going to support that.”

Lost in all of the obvious concern about Russia is the fact that Trump is pushing an extremely, extremely right-wing, reactionary agenda. … I’m not going to support that. – Sen. Bernie Sanders (I-VT)

Of the 35 nominees included in POLITICO’s analysis, Gillibrand supported just one, Warren backed two and Booker and Harris gave their endorsement to three. Sanders, who ran for the Democratic presidential nomination last year, supported three Trump nominees while Merkley backed six. (Harris and Sanders both missed a confirmation vote each that could affect their totals.)

POLITICO’s analysis did not consider three nominees confirmed unanimously: Secretary of Veterans Affairs David Shulkin and two members of the U.S. Sentencing Commission.

Few Democrats have the platform to showcase their opposition to Trump like members of the Senate, who have the power to filibuster legislation and give their imprimatur to hundreds of nominees across the administration. But this year’s resistant bloc of liberals stands out even among a deeply recalcitrant Democratic Caucus, which waged the first partisan filibuster of a Supreme Court candidate in history and launched numerous other wars over nominees — winning plaudits from a restless base that’s urging lawmakers to oppose Trump at every turn. In an interview, Harris also dismissed any notion that her recurring “no” votes have anything to do with potential national aspirations.

“Literally, I could go down the list in terms of on merit and on the issues, why I voted the way I did,” the freshman senator said. “There is a reason why the United States Senate has a responsibility to confer and to accept or reject the president’s nominees to the Cabinet and to the administration. And I take that very seriously.”

The half-dozen Democrats in this year’s hell-no caucus rejected the vast majority of even the 14 Trump nominees who snagged votes from most others in their caucus. Only two nominees won over a majority of the group: U.N. Ambassador Nikki Haley, who got votes from all but Sanders; and Defense Secretary James Mattis, backed by all except Gillibrand, the Democrats’ most frequent nomination naysayer.

Sen. Kirsten Gillibrand (D-NY).

No senator questioned the law enforcement credentials of Rosenstein, a veteran U.S. attorney whose profile rose when he was handed the reins of the federal investigation into potential collusion between Russian officials and the Trump campaign. But six Democrats nonetheless voted against him, with most arguing that Rosenstein’s then-refusal to commit to appointing a special counsel should disqualify him from being deputy attorney general. Democratic unease with Rosenstein spread when his role in the abrupt firing of FBI Director James Comey was disclosed earlier this month.

“A lot of my concerns about him are playing out, but again, this is not a time for rash judgments,” Booker said the day after Comey was fired. Rosenstein has since tapped former FBI chief Robert Mueller as special counsel in the Russia probe, earning back a measure of goodwill among Democrats.

Of his opposition to Trump officials, Booker insisted he would not “rubber stamp” confirmations and argued the nominees have been unqualified, favored the administration’s “backward and even unconstitutional actions,” or have “simply been unwilling to stand up for what’s right.”

As for Gillibrand, the New York senator gave a simple explanation for her “no” votes to New York magazine earlier this year: “If they suck, I vote against them. If they’re worthy, I vote for them.” An aide said Gillibrand had no broader strategy to vote against nearly all Trump nominees — a steady stream of opposition that began when she was the sole senator to vote against Mattis on the first day of Trump’s presidency.

The scope of that resistance is deeply frustrating to Republicans, who have had to burn up floor time on nominees that Democrats ultimately can’t stop from getting confirmed. Some in the GOP recall a similar wave of hard-line opposition to President George W. Bush’s nominees as the 2008 presidential primaries heated up, with then-Sens. Barack Obama, Joe Biden and Hillary Clinton joining other White House hopefuls in filibustering Supreme Court Justice Samuel Alito’s nomination.

“When President Obama, Vice President Biden and Secretary Clinton were in the Senate, they often voted against nominees — even though those nominees had wide bipartisan support — as a means of protecting themselves from their base and setting themselves up for a primary,” said Don Stewart, a spokesman for Senate Majority Leader Mitch McConnell (R-Ky.). “The attempted filibuster of Justice Alito is a perfect example. They knew their filibuster would fail, but they were worried about losing support in the primary.”

But the six liberals’ conspicuous contrarianism against Trump’s nominees place them among a caucus that is broadly leaning more to the left as the White House makes little attempt at outreach to the minority.

The 10 Senate Democrats who face reelection next year in states Trump won have mostly voted with their party against his most controversial nominees and the GOP’s regulatory rollbacks. However, those Trump-state Democrats have also almost always joined the majority of their party in approving second-tier nominees. Sen. Sherrod Brown of Ohio, a liberal stalwart, backed 12 of the 14 Trump picks that won over a majority of his fellow Democrats.

Even though the six Democrats publicly separate their sweeping rejection of Trump nominees from any desire to appeal to their party’s emboldened base, liberal activists are pleased by the Senate’s crop of hardy “no” voters.

“I would say good on them,” MoveOn.org Executive Director Anna Galland said in an interview. “Democrats who stand up and fight fiercely are closest to meeting the demand of the movement in the streets.”

Democrats who stand up and fight fiercely are closest to meeting the demand of the movement in the streets. – MoveOn.org Executive Director Anna Galland

Warren’s office did not return a request for comment on her record of opposition. She notably took heat from liberals earlier this year for backing Housing and Urban Development Secretary Ben Carson in committee, before opposing him on the floor. Warren later attributed her turnabout to contentious executive orders Trump signed in the interim, as well as the resignation of his scandal-plagued national security adviser, Michael Flynn.

Merkley also portrayed his “no” votes as a statement on the Trump administration’s broader actions.

“Right now, I feel that we are on the brink of a constitutional crisis. We’re staring into the abyss, and we shouldn’t just be treating it as business as usual, confirming people,” Merkley said. “So I’m lodging a bit of a protest against the Republicans’ desire to just pretend there is no issue here.”

Copy edited by Sushant Sagar. Produced by Lily Mihalik.