WASHINGTON — A long-running federal investigation into a former White House counsel in the Obama administration is reaching a critical stage, presenting the Justice Department with a decision about whether to charge a prominent Democrat as part of a more aggressive crackdown on illegal foreign lobbying.

The case involving the lawyer, Gregory B. Craig, was transferred in January from federal prosecutors in New York to those in Washington. The previously undisclosed move was driven by Justice Department officials in Washington, and reflects an eagerness within the department to prosecute violations of lobbying laws after the special counsel, Robert S. Mueller III, focused on foreign influence in his investigations.

A decision about whether to prosecute Mr. Craig, who was White House counsel for President Barack Obama during his first year in office, is expected in the coming weeks, people familiar with the case said. The investigation centers on whether Mr. Craig should have disclosed work he did in 2012 — while he was a partner at Skadden, Arps, Slate, Meagher & Flom — on behalf of the Russia-aligned government of Viktor F. Yanukovych, then the president of Ukraine.

The work was steered to Mr. Craig by Paul Manafort, who was then a political consultant collecting millions of dollars from clients in former Soviet states. Mr. Manafort, who went on to become President Trump’s campaign chairman in 2016, was sentenced this month to seven and a half years in prison on charges brought by Mr. Mueller’s team related to obstruction of justice and violations of banking, tax and lobbying laws stemming from his work in Ukraine.