Al Franken. Joe Raedle/Getty Images

Democratic Sen. Al Franken of Minnesota refused to sign off on President Donald Trump's nominee for a circuit court vacancy.

The move represents a roadblock for Trump as he tries to rapidly reshape the federal courts.

Franken said he was concerned about the nominee's conservative record.

Democratic Sen. Al Franken of Minnesota on Tuesday refused to sign off on President Donald Trump's nomination of a judge for a vacancy on the 8th US Circuit Court of Appeals, highlighting perhaps the biggest challenge Trump faces as he tries to remake the federal judiciary.

Franken said he could not support the nomination of Minnesota Supreme Court Judge David Stras, nominated by Trump in March, and would not be returning a blue slip to the Senate Judiciary Committee.

The "blue slip" process is a tradition in which US senators can give their blessing to judicial nominees from their state. It is not entirely uncommon for home-state senators to deny providing the blue slip for a judicial nominee — several of President Barack Obama's judicial nominees met the same fate — but the move comes as Trump tries to rapidly fill more than 130 federal vacancies.

"As I have familiarized myself with Justice Stras's record — not just his past decisions, but his professional experience and past statements — I have grown concerned that, if confirmed to the federal bench, Justice Stras would be a deeply conservative jurist in the mold of Supreme Court Justices Clarence Thomas and Antonin Scalia, justices who the nominee himself has identified as role models," Franken said.

The obscure move from Franken emphasizes the roadblocks Trump could face in landing Democratic support for his judicial nominees in the roughly 30 states with at least one Democratic senator.

While the denial of a blue slip does not legally restrict a judge from being approved, Glenn Sugameli, an attorney who is an expert on judicial nominations, told Business Insider in an email that "no circuit court nominees have been confirmed over objection of one (or two) home state senators — including under Obama."

Donald Trump. Shawn Thew-Pool/Getty Images

In his early judicial blitz, during which he has nominated far more district court and circuit court judges than Obama, Trump has largely avoided making nominations in states that are represented by at least one Democrat.

Roughly 80% of his 36 nominations to district or circuit courts are from states that are represented by two Republican senators. Just two of the 36 nominations hail from states with two Democratic senators.

At the same point in his presidency back in 2009, Obama had nominated nine people for vacant district court seats and seven candidates for circuit court openings. Of those, 56% were from states represented by two Democratic senators — meaning 44% faced the scrutiny of at least one member of the opposition party before they could be advanced in the Senate.

The intent of the blue-slip process is to have a more bipartisan consensus on judges who will serve in or represent a senator's home state when the president is of the opposition party, encouraging communication between the White House and home-state senators before a nomination.

A White House official told Business Insider in July that the Trump administration was "working with and extensively consulting all senators nationwide in order to complete the nomination process." The official added that the White House was "committed to filling all the ... vacancies as quickly as possible."

Sugameli said the past two chairmen of the Senate Judiciary Committee — Democratic Sen. Patrick Leahy of Vermont and Republican Sen. Chuck Grassley of Iowa — blocked every Obama nominee who was unable to obtain both blue slips.

Sen. Amy Klobuchar, a fellow Minnesota Democrat who did not make a formal announcement about whether she would provide the blue slip for Stras, said in a statement following Franken's move that she was concerned Trump would nominate someone from an 8th Circuit state with two Republican senators — such as Arkansas, Nebraska, Iowa, or South Dakota — instead of finding a consensus judge from her home state. She also warned against changing the practice.

"Changing this policy would have serious ramifications for judicial nominations in every state in the country," she said. "Given this important policy, and given Sen. Franken's view that Justice Stras should not be allowed a hearing in the Senate, the White House will need to provide additional names for the Eighth Circuit position."

Sen. Dianne Feinstein of California, the ranking Democrat on the Judiciary Committee, also expressed hope that the practice would be continued, and she showed support for Franken's move.

"It's the prerogative of home-state senators to evaluate potential federal judicial nominees and determine whether or not they are mainstream and well-suited to hold these important positions of public trust, which have real-world consequences for their constituents," she said in a statement. "The purpose of the blue slip is to ensure consultation between the White House and home-state senators on judicial nominees from their states. I expect the committee to honor Sen. Franken's decision not to return a blue slip, as was always done when Republican senators didn't return blue slips on President Obama's nominees."

The sheer speed with which the Trump administration is nominating judges to fill the many vacancies has "created some problems," Carl Tobias, a University of Richmond law professor who is an expert on judicial selection, told Business Insider in July.

Namely, he said he did not think enough consultation with home-state senators had taken place.

"The home-state senators really have an important role to play," he said. "And it's one of the last pieces of patronage they have. So they're really defensive about that."