Former Attorney General Eric Holder and his law firm, Covington & Burling LLP, will conduct vetting of potential running mates for Vice President Kamala Harris’ campaign for the U.S. presidency, according to two sources familiar with the matter.

Harris is running for the Democratic presidential nomination after President Joe Biden announced on Sunday he was stepping aside.

A campaign spokesperson for Harris declined to comment.

Holder did not respond to a request for comment.

Harris is aiming to wrap up the Democratic nomination in the coming days and the selection of a running mate is one of the most important early decisions she can make.

Biden selected her as his running mate in 2020 and said in his letter announcing his candidacy withdrawal on Sunday that it was the best decision he had made.

Holder served as attorney general for President Barack Obama for six years and was the first African American to hold the country’s top law enforcement job.

His involvement in the vetting process may reassure Democrats that a longtime political and legal hand is helping to ensure Harris’ pick, should she win the nomination, is free of major conflicts. Any choice needs also to be suited to help her beat Republican Donald Trump and potentially assume the presidency someday if she were to triumph in November but then at some point be unable to complete her duties.

Potential names that have been floated as possible running mates for Harris include Pennsylvania Governor Josh Shapiro, Michigan Governor Gretchen Whitmer, U.S. Senator Mark Kelly of Arizona, Kentucky Governor Andy Beshear and North Carolina Governor Roy Cooper.

Holder is a civil rights advocate and chairs the National Democratic Redistricting Committee, which works to oppose Republican gerrymandering – efforts by politicians to redraw voting maps in U.S. states to give their party an advantage. The group argues that Republicans have used the power of “redistricting” to disenfranchise voters, especially people of color.

Harris is also a voting rights advocate and took on that issue as one of her portfolios when she first became Biden’s vice president, but her efforts did not lead to major reforms.