Stripped of the procedural right to stop President Donald Trump's judicial nominations, Senate Democrats are trying to slow what they say is a Republican power move – and escalating a partisan holy war over the federal judiciary – with a blue piece of paper.

Late last week, a trio of Democratic senators sidetracked two of Trump's judicial nominees by withholding blue slips, a traditional but powerful Senate consent form required for confirmations to proceed. The senators say it's pushback against a White House that has nominated extremely conservative jurists from their states without seeking their counsel, and GOP leadership, which has fast-tracked the process to fill some 150 judicial vacancies former President Barack Obama left behind.

But Senate Majority Leader Mitch McConnell – who took away the Democrats' filibuster in March after they attempted to scuttle the confirmation of Supreme Court Justice Neil Gorsuch – hinted Wednesday that the Democrats' blue-slip strategy could soon meet a similar fate.

"My personal view is that the blue slip, with regard to circuit court appointments, ought to simply be a notification of how you're going to vote, not the opportunity to blackball," McConnell told The New York Times. He said he favored keeping the custom for lower-level district court judges.

Senate Minority Leader Charles Schumer reportedly has asked to meet with McConnell and Senate Judiciary Chairman Charles Grassley, an Iowa Republican, to try and de-escalate the confrontation with a simple warning: What goes around, comes around.

"Preserving some of the minority's power in the Senate has broad support," Schumer told The Times, "because every one of us knows we're probably going to be [out of power]" at some point over a six-year Senate term.

The confrontation, and McConnell's ominous warning to Democrats, is an example of what's at stake: Control of the federal bench, arguably the most consequential spoils in partisan control of government. Besides creating a feeder system for Supreme Court nominees, the federal appellate court in particular has the power to shape the national social and legal landscape long after the Senate majority and the Oval Office has changed hands.

Exhibit A: Nine months into his first term, Trump has dozens of vacancies in the upper echelons of his administration, from zero deputy secretaries at the Department of Housing and Urban Development to no second- or third-in-command for Education Secretary Betsy DeVos.

Yet Trump and the Senate have confirmed what some say is a record number of federal judges – five – for a first-year president.

Exhibit B: The ongoing legal wrangling over Trump's ban on immigrants from Muslim-majority countries.

Since the president signed an order implementing the ban, national-security conservatives and pro-immigrant civil libertarians have done battle before a range of federal judges appointed by Trump's predecessors, including former Presidents Bill Clinton, George W. Bush and Barack Obama. The Supreme Court – with a newly-reconstituted conservative majority, anchored by Gorsuch, a former appellate judge – will get the last word on the matter.

Dating back a century or so, the blue slips are mostly traditional and not bound by Senate rules, but the Senate is nothing if traditional. By withholding the document, senators can stall a nomination indefinitely – leverage to bring the majority, or even the White House, to the negotiating table for legislation or a more acceptable nominee.

The Democrats' demands in this case: Trump must consult with them first before making any more judicial nominations, and Grassley must stop the lightning-round confirmation process.

Not surprisingly, the White House doesn't see it that way.

"The administration has been seeking consultation from home-state senators for months – even as senators frequently fail to return our calls, don't respond to our inquiries and otherwise avoid our constant overtures," a Trump administration official told The Washington Times.

Irate conservatives accuse Democrats of abusing Senate tradition to create an illicit, back-door filibuster and further polarizing Congress. Grassley, a Senate traditionalist who pledged to respect the blue-slip process when he rose to judiciary committee chairman, reportedly is considering changing Senate rules to shut down the tactic.

But there's no sign the Democrats will back down.

Late last week, Oregon Senators Jeff Merkley and Ron Wyden told the White House that they won't approve the nomination of Ryan Bounds, a conservative federal prosecutor whom Trump wants for an appellate court vacancy. Merkley and Wyden contend they won't follow tradition because Trump didn't consult with them about Bounds, and the nominee's record hasn't been carefully scrutinized.

"As we have made clear," the senators wrote in a blunt letter to White House counsel Don McGahn, "we do not intend to return our blue slips for Ryan Bounds or any other nominee that has not been selected through our judicial nomination process."

The letter arrived just days after Sen. Al Franken of Minnesota declared he's withholding a blue slip for Justice David Stras, a state supreme court judge whom Trump has nominated for another appellate judgeship. Franken believes Stras' jurisprudence is a bad fit for a court that's already very conservative and has just one woman out of seven justices.

"The president should be seeking out judges who bridge the issues that divide us," Franken said in a statement last Tuesday. "I fear that Justice Stras' views and philosophy would lead him to reinforce those divisions and steer the already conservative Eighth Circuit even further to the right."

The White House wasn't happy, blasting Franken's blue-slip move as obstructionist politics at its worst. "He deserves a chance to have a hearing so the American people can see that for themselves," a White House spokesperson told The Star Tribune of Minneapolis.

The administration, however, is getting close-air support from Judicial Crisis Network, a deep-pocketed conservative organization. On Wednesday, JCS announced it has launched an expensive new video ad campaign slapping Franken for blockading Stras.

"So, is Al Franken jealous of someone more popular? Someone with bipartisan support?" a narrator asks in the 30-second spot, urging viewers to call the senator's office. "Tell Senator Franken – stop being petty – grow up. Confirm Justice David Stras."

But progressive groups want Democratic senators to stop hard-right nominees they say are vetted solely by far-right think tanks and are out of the mainstream of judicial thought. And leaders on the left say they want get-to-know-you confirmations, not rapid-fire speed dating.

"I do think that there's a very concerted effort to put extremely conservative nominees on the federal bench," says Christopher Kang, a former White House counsel under President Barack Obama. "The evidence of that is how many nominees that President Trump has made already who come from the (extremely conservative) Heritage Foundation-authored and -approved short list."

Kang, a member of the American Constitution Society's board of directors, says it matters because most of the nominees get lifetime appointments, they get to decide on issues that affect everyone and their lifetime tenure will far outlast the Trump presidency.

David Super, a Georgetown University constitutional law professor, says blue slips are usually used as a bargaining chip, a way to get bipartisan cooperation on judicial nominees. If a senator withheld a blue slip, it usually led to negotiation between the Senate and the White House for a more suitable nominee.

The slips "were a way of avoiding wasting Senate [time] on nominations that are going to be filibustered" by one party or the other, Super says.

He pointed to legendary New York Sens. Daniel Patrick Moynihan, a Democrat, and Alfonse D'Amato, a Republican, colleagues in the 1990s who had reached a gentleman's agreement on judicial nominations. If a Democratic president was in office, Super says, Moynihan would get to approve two judicial picks and D'Amato would get just one; if a Republican was in office D'Amato got two picks to Moynihan's one.

"It led to a lot of moderate judges getting appointed" to New York's federal courts, Super says. "They both agreed to submit the blue slips on the others' picks" and ended up getting moderates on the bench as a result.

Veteran senators in the majority have also used blue slips as a teachable moment, Super says, for new presidents who pushed hard to move the court to the right or to the left. Republicans used blue slips to thwart Obama's nominees, and Democrats pushed back on former President George W. Bush to similar effect.

The confrontations going on in the Senate, however, have the potential to ratchet up already-intense hostilities between Democrats – still seething over Republicans' unprecedented decision to block Obama from filling a Supreme Court vacancy and installing a center-left majority – and Republicans, who remember that then-Majority Leader Harry Reid stripped them of the filibuster as punishment for blocking Obama's lower-court judicial nominations.

If both sides don't find a way forward, "it could lead to a further partisan break," Super said.

Though the GOP has the upper hand, Super says, there's a risk if Grassley does decide to go nuclear with committee Democrats. "If you give the minority too little, there is a danger they could be more obstructionist in a lot of other ways" to sidetrack the GOP agenda, he says.

While Grassley may be in the majority now, Super says, he surely knows that the shoe Democrats are wearing could soon be on the Republicans' collective foot. The X factor, he says, is Trump, and whether Grassley, or McConnell, is willing to use political muscle on his behalf.

"Donald Trump will never be a member of the Senate minority in the future. Charles Grassley has been, or he may be in the future," Super says.