Adherence to the custom has waxed and waned, depending on the views of Senate leaders. But the rule was strictly observed during the Obama administration, and GOP opposition to President Barack Obama's nominees partly explains why Trump entered office with more than 120 judicial vacancies to fill.
Removing the blue-slip obstacle would make it much easier for Trump's choices to be confirmed. Although Trump and Senate Republicans have clashed early in his presidency, they agree on the importance of putting conservatives on the federal bench.
Senate Republicans changed the chamber's filibuster rule in April to confirm Neil M. Gorsuch as a Supreme Court justice and applauded Trump's first round of nominations for federal circuit courts of appeals and district courts. His choices were drawn in part from the recommendations of conservative groups such as the Federalist Society and the Heritage Foundation.
The Senate acted Thursday on Trump's first appeals court nomination, elevating U.S. District Judge Amul Thapar of Kentucky to the U.S. Court of Appeals for the 6th Circuit, which covers Kentucky, Michigan, Ohio and Tennessee. Thapar was confirmed 52 to 44 on a party-line vote, with four Democrats not voting. Thapar's nomination did not raise blue-slip concerns, because both of Kentucky's senators are Republican and Thapar is a favorite of Senate Majority Leader Mitch McConnell (Ky.).
Conservative groups have urged McConnell and Senate Judiciary Committee Chairman Charles E. Grassley (R-Iowa) to loosen the blue-slip rules — especially on nominees to regional appellate courts — and Republicans have warned Democrats that uncompromising opposition to Trump's nominees could trigger a change.
"Everybody agrees that blue slips on federal district judges are appropriate where the districts are contained within a state and that's been the tradition," said Sen. John Cornyn (Tex.), the chamber's second-ranking Republican. "My sense is that we're going to establish a pattern where a blue slip at the circuit court level is an expression of advice, but is not determinative as to whether that judge will be confirmed or not."
Democrats say that would be a substantial reworking of the current rules and inconsistent with Grassley's pledge to retain the blue-slip process no matter which party captured the White House last year.
"Eliminating the blue slip is essentially a move to end cooperation between the executive and legislative branch on judicial nominees, allowing nominees to be hand-picked by right-wing groups," Sen. Dianne Feinstein (Calif.), the ranking Democrat on the Judiciary Committee, wrote in a memo this week.
She pointed out that the vacancy for which Thapar is nominated exists only because McConnell refused to return a blue slip for Obama's nominee, Kentucky Supreme Court Justice Lisabeth Tabor Hughes. The seat has been vacant since 2013, and Tabor Hughes never received a hearing because blue slips were not returned.
Christopher Kang, who advised Obama on judicial nominations, said 17 of the president's picks did not receive hearings for that reason, killing the nominations.
But the impact was even greater than that, because Obama gave up on trying to find nominees in some states, such as Texas, with two Republican senators. One vacancy on the U.S. Court of Appeals for the 5th Circuit, which covers Louisiana, Mississippi and Texas, has been open for five years.
"There's no question that the blue-slip process greatly influenced the way President Obama chose nominees and whether they received hearings," Kang said.
Now, liberal groups that denounced Republican stalling over Obama nominees — his picks languished longer before action than did President George W. Bush's — are urging Democratic senators to use blue slips to block Trump nominees. Conservative groups until recently defended the process as time-honored.
Because it is a custom of the Senate and not a legal requirement, the blue-slip process has been interpreted different ways over the years.
Until the 1950s, objections by home-state senators did not hang up judicial nominations, according to a Congressional Research Service report. But during the civil rights movement, Southern lawmakers demanded more say over the judges from their states and succeeded in winning more deference. This proved temporary. By the 1980s, this deference was again on the wane. Then, during Obama's tenure, senators were yet again able to assert their power to block judicial nominees.
"The blue-slip process was always intended to ensure consultation, and Grassley fully expects senators to continue to abide by that tradition," said Taylor Foy, a Grassley spokesman.
But Democratic aides noted that Grassley didn't always feel this way and that he is now reneging on vows made to Democrats that he would preserve the blue-slip system no matter who was elected president in 2016. In 2015, he wrote in the Des Moines Register that "I appreciate the value of the blue-slip process and also intend to honor it."
Russell Wheeler, a Brookings Institution scholar who follows judicial nominations, said allegiance is situational.
"All of a sudden, Republicans are discovering the blue-slip process can be abused, when in fact they've been abusing them to get all of these vacancies for Trump to fill," he said.
Senate Minority Leader Charles E. Schumer (D-N.Y.) blasted Republicans for departing from tradition, saying in a statement that "The Constitution requires the advice and consent of the Senate, not right wing interest groups, on the president's judicial nominees." But there's very little that Democrats can do to stop Trump from nominating conservative judges and the Republican-controlled Senate from confirming them. It was Democrats, when they controlled the Senate in 2013, who changed the rules barring filibusters on judicial nominees, which required 60 votes to move forward on a nomination.
Sen. Sheldon Whitehouse (D-R.I.), a former U.S. attorney and state attorney general who is among the most liberal members of the Judiciary Committee, said he doubts that Grassley "would want to dismantle that long-standing Senate prerogative just to cater to the extremist impulses of this particular administration."
Given that appellate court seats are historically connected to individual states, "they should continue to be able to make recommendations," Whitehouse said. "I think that would be a really dumb mistake to make just to appease the far right because it would have lasting consequences that would diminish the Senate both for Republican and Democrat senators."
Leonard Leo, who is advising the White House on judicial nominations, said the potential of changing the blue-slip process has not altered the tradition of consulting with home-state senators.
"The administration has engaged in as vigorous a consulting process as I've ever seen, and is doing anything they can to hear out Democratic senators," he said.
Trump has nominated 10 judges in addition to Thapar, and two of them are from states represented in the Senate by Democrats. Minnesota Supreme Court Justice David R. Stras and Michigan Supreme Court Justice Joan L. Larsen were on Trump's list of potential U.S. Supreme Court picks. Stras is nominated to the U.S. Court of Appeals for the 8th Circuit, and Larsen to the 6th Circuit.
Michigan's Democratic senators, Debbie Stabenow and Gary Peters, said they were "informed" of Trump's intention to nominate Larsen and have made no judgment about her. "I will continue to listen to public input and consult with Michigan's legal community to ensure that our state is served by highly qualified, fair and impartial judges that put the people of Michigan first," Stabenow said in a statement.
Minnesota's Democratic senators, Amy Klobuchar and Al Franken, also have been noncommittal about Stras.
But they're insisting on maintaining the deference they now receive.
"It's customary that the blue-slip process applies equally to both district and circuit court nominees — and Republicans certainly operated that way during the Obama administration," Franken said, adding, "The committee should continue this custom and not change it simply because there's a new president in the White House."