A meeting between the Vice-President’s aides and D.N.C. staffers may indicate that Biden, pictured earlier this week, is seriously mulling a Presidential run. Photograph by Olivier Douliery / ABACA / SIPA via AP Images

Joe Biden has taken another step toward entering the Presidential race.

Representatives of the Vice-President held a meeting this week with Democratic National Committee staffers. They briefed Biden’s aides on arcane but crucial rules that the Vice-President would need to understand if he decides to run, according to a D.N.C. official.

It was the most significant sign the source had seen to indicate Biden’s intentions. “I think it means he’s running,” the source said.

The D.N.C. has held similar meetings for representatives from the five declared Democratic candidates: Hillary Clinton, Bernie Sanders, Jim Webb, Martin O’Malley, and Lincoln Chafee. The D.N.C. offered the meeting to Biden earlier this year, and the party committee was scheduled to brief his aides back in June, but that meeting was cancelled.

When the meeting was finally held this week, D.N.C. staffers walked Biden’s representatives through the primary calendar, filing deadlines, the mechanics of ballot-access issues, and the complicated details of the party’s state-by-state selection process for delegates and super delegates. The session included a level of detail that would only be of interest to a candidate who is serious about running. For example, the briefing included information about issues such as whether a particular primary state requires a candidate to send a letter to the Secretary of State in order to get on the ballot, or circulate a petition, or pursue some other method.

But, while the briefing is considered an important step for any candidate who is serious about entering the race, how it will affect Biden’s decision is unclear. It may simply be that Biden is still gathering the necessary information to help him make a final determination. And the details of the complicated process of winning enough delegates to secure his party’s nomination might convince Biden that the window to enter the race has passed.

The D.N.C. source, who was briefed on the meeting, said that the information conveyed seemed eye-opening for Biden’s aides. “They probably thought they had a lot longer,” the source said. “The deadlines for qualifying on the ballots for key states haven’t passed yet, but are fast approaching.”

“D.N.C. staff offers all Democratic Presidential candidates and potential candidates briefings on the ballot and delegate process,” said a D.N.C. spokesperson, declining to confirm or deny whether D.N.C. staff briefed Biden’s representatives this week.

The rumblings in Democratic circles about whether Biden may be leaning toward running seem to be impacting the Clinton campaign. Yesterday, Clinton announced that she no longer supported the Trans-Pacific Partnership, the trade deal that, in 2012, she said “sets the gold standard in trade agreements to open free, transparent, fair trade, the kind of environment that has the rule of law and a level playing field.” Additionally, in “Hard Choices,” Clinton’s 2014 memoir of her tenure as Secretary of State, she wrote that the T.P.P. would be a “signature economic pillar of our strategy in Asia.”

The trade deal is vehemently opposed by labor unions, whose support Biden would count on if he enters the race. As a sitting Vice-President, it would be far more difficult for Biden to oppose the T.P.P., one of Obama’s top remaining priorities, and on Tuesday, Biden’s spokesperson confirmed that he supports the bill and will help pass it on the Hill.