(Reuters) - The U.S. Supreme Court declined on Friday to halt a Financial Industry Regulatory Authority proceeding to expel one of its members, broker-dealer Alpine Securities, in a case challenging the constitutionality of the Wall Street's self-regulator's enforcement power.

Chief Justice John Roberts denied Alpine's request to stop the FINRA proceeding, while the Supreme Court considers whether to hear the broker-dealer's appeal of a lower court ruling that allowed the non-governmental self-regulatory organization's enforcement action to move forward.

FINRA has weighed the expulsion of Salt Lake City-based Alpine, accusing it of stealing more than $54.5 million from customers by charging excessive fees and misusing their investments.

FINRA, Wall Street's industry-funded self-regulatory organization, is responsible under federal law for supervising its firms. FINRA opposed Alpine's request to stay its enforcement proceeding. President Donald Trump's administration also opposed a stay.

Alpine was appealing a November 22 decision by the U.S. Court of Appeals for the District of Columbia Circuit.

That court ruled that FINRA cannot expel member firms in expedited proceedings without obtaining review by the U.S. Securities and Exchange Commission, a federal regulatory agency.

The D.C. Circuit decided that a lack of SEC review likely violated a constitutional principle known as the nondelegation doctrine, a constitutional doctrine that requires that a private entity such as FINRA that exercises congressionally delegated regulatory authority must be supervised by a government actor such as the SEC.

But the appeals court nonetheless let FINRA continue its enforcement proceeding against Alpine, saying it would not cause the kind of irreparable harm Alpine could face if later expelled, such as going out of business.

In its Supreme Court appeal, Alpine said it faced irreparable harm from having to defend itself in FINRA's enforcement proceeding, which it called unconstitutional.

It also said a stay would preserve the status quo while the Supreme Court considers several other nondelegation cases.

One concerns whether Congress illegally granted the Federal Communications Commission power to oversee an approximately $9 billion fund to help rural and low-income residents obtain telephone and broadband access.

A Supreme Court decision in that case is expected by the end of June.

In opposing a stay, FINRA said no court has found that having to defend oneself before an adjudicator challenged as unconstitutional causes irreparable injury in cases such as Alpine's.

FINRA also said Alpine's appointment and removal claims have been rejected by every appeals court that has considered whether to extend protections to private parties.

The Trump administration, meanwhile, said halting FINRA's proceeding would "frustrate the political branches' (and the public's) goal of encouraging self-regulation of the securities industry," and the "strong congressional and public interest in ensuring that securities markets remain trustworthy and sound."