WASHINGTON – Senate Minority Leader Harry Reid has been actively reviewing Massachusetts rules for filling a US Senate vacancy, another indication of the seriousness with which Democrats are gaming out the possibility of Elizabeth Warren joining likely presidential nominee Hillary Clinton’s ticket.

The upshot of Reid’s review is that Senate Democrats may have found an avenue to block or at least narrow GOP Governor Charlie Baker’s ability to name a temporary replacement and prevent the Senate from flipping to a Democratic majority if Warren were to leave the chamber. That suggests the issue is not as significant an obstacle as Reid previously feared.

Pieces of the legal guidance given to Reid were shared with the Globe by a person close to Reid who is familiar with the guidance.

“Reid sees a number of promising paths to making sure that Democrats keep Warren’s seat and is very open to her being selected,” said this person, speaking on the condition of anonymity to discuss internal deliberations.

Warren is a frequently mentioned figure in the capital’s fevered vice presidential speculation. Clinton’s campaign, while not explictly confirming that Warren is under consideration, has said its list of finalists for the pick will include at least one woman.

Warren, who is highly popular among Democratic liberals, would provide an immediate boost to Clinton by shoring up her left flank after the prolonged and divisive fight with Vermont democratic socialist Senator Bernie Sanders for the nomination.

But for Senate Democrats, a Clinton-Warren ticket is not without risk. If Clinton were to win in November, the Massachusetts senator’s departure for the vice presidency could threaten Democrats’ hopes of recapturing the Senate majority.

Reid recently gave voice to those concerns and said he would not want Clinton to pick a vice presidential nominee from a state with a Republican governor — such as Massachusetts.

But subsequently Reid commissioned a review by Washington election law attorney Marc Elias (who is also the general counsel to the Clinton campaign, and has advised Warren on legal matters in the past). The review only focused on Massachusetts, and Reid did not conduct such a follow-up review on any other state, according to the person source close to Reid.

One key area Reid and his advisers are examining is how long Baker would have to fill Warren’s seat with a temporary replacement.

In the event of a Senate or House vacancy, Massachusetts currently requires a special election to be held within 145 to 160 days. In the interim, the governor has the authority to appoint a successor. But Reid’s team has identified a portion of the law that allows an officeholder to start the special election clock by filing a resignation letter, but also announcing an intention to vacate the seat at a later date.

In theory, Warren could file such a letter 145 days before the Jan. 20, 2017 inauguration and successfully block Baker from picking any temporary replacement. But that would expose Warren to a potentially awkward position. If Clinton lost the November election and Warren wanted to keep her Senate seat, she would have to make the politically difficult decision of rescinding her planned resignation — or run for an open seat that she created.

A more likely scenario would be that Warren would start the clock ticking for a special Massachusetts ballot only if Clinton won, with an intent-to-resign letter dated the day after the Nov. 8 national election.

That would give Baker’s temporary appointee less than three months to serve between Inauguration Day and the special election. While that might prevent a Democratic majority from taking over in January, the damage, from a Democatic perspective, might be limited to a short period of time until Massachusetts’s Democrat-leaning electorate went to the polls to elect a senator in a special election.

Over the past two years Warren has pledged repeatedly to serve out her full Senate term, which ends in January 2019. That could change, though, if she is named the vice presidential nominee.

Warren’s office declined to comment on the speculation over possibilities of her as vice president, and what would happen to her seat in the event of a vacancy. A spokeswoman for Baker also declined to weigh in. A vacancy would put Baker — a Republican governor in a blue state who has sought to avoid national GOP politics — in a sensitive position, particularly if the US Senate majority hung in the balance.

“Governor Baker is focused on priorities in state government, such as passing a budget, improving the MBTA, and fighting the opioid crisis in the Commonwealth, not presidential politics,” said Baker spokeswoman Elizabeth Guyton.

A spokesman for Secretary of State William Galvin concurred with the idea that Warren could file a letter of resignation — triggering an election — but could make the actual resignation effective at a later date, preventing Baker from filling a temporary replacement for an extended period of time.

“Presumably if she wanted to do it, then yes, that would cut down on the time,” said Brian McNiff, a spokesman for Galvin.

With Donald Trump’s victory over his primary opponents in the GOP nominating contest, Democrats are increasingly confident they can win back the Senate, which is currently a 54-to-46 GOP majority. A Democrat would be favored to win a Massachusetts special election, but Republican Scott Brown’s 2010 upset victory in the campaign to replace the late Senator Edward M. Kennedy remains a relatively fresh memory.

Reid, who is leaving the Senate in January, views himself as a peacemaker between Clinton’s camp and the progressive wing of the party that Sanders has ignited. He has grown increasingly concerned over the divisiveness in that race.

Reid last week had initially dismissed the idea of Clinton appointing a US senator from a state that has a Republican governor who could pick a successor, a scenario that applies to Warren as well as to Sherrod Brown of Ohio and Cory Booker of New Jersey.

“If we have a Republican governor in any of those states the answer is not only no, but hell no,” Reid told MSNBC on May 23. “I would do whatever I can and I think most of my Democratic colleagues here would say the same thing.”

Reid mentioned Lloyd Bentsen leaving his US Senate seat in Texas to become US treasury secretary in 1993 (Bentsen in 1988 ran simultaneously as Michael Dukakis’ nominee and for reelection in Texas; because they lost the presidential campaign he stayed in the US Senate).

“We have never recovered from that,” Reid said. “Had we not gone along with that we could still have a Democratic senator from Texas. So no, I would yell and scream to stop that.”

The prospect of a Warren vice presidential nod has some detractors within the party. On Friday, Howard Dean, who in 2004 captured the progressive fervor, suggested that Warren, at age 66, is too old to be the nominee.

“I strongly beleive we ought to have someone under 50 on the ticket,” Dean said on MSNBC. “Ideologically, I think Elizabeth Warren is terrific. I just would like to see a new generation brought into Democratic Party politics. There are plenty of well-qualified young people who could be vice president.”

Massachusetts has a convoluted history with its election laws around filling a US Senate vacancy, as well as a habit of changing the laws based on raw partisanship.

The last two Senate vacancies in Massachusetts — one caused by the death of Kennedy, the other by John Kerry’s appointment as secretary of state — occurred while Deval Patrick, a Democrat, was governor.

But in 2004, when Kerry was the Democratic presidential nominee, the law would have allowed then-governor Mitt Romney to name a successor.

So the Democrat-controlled Legislature changed the law, calling for a special election instead. As a way to prevent Romney from naming a temporary successor, the law also ensured that the seat stay vacant until the special election was held.

It was also at this time that the Legislature added language allowing for a letter of resignation to be filed, but not making it effective until a later date.

In 2009, with Kennedy ill and Democrats barely holding onto their filibuster-proof majority in the US Senate, Democrats worried about going for a long stretch without a Democrat who could vote on a landmark health care law. So the Legislature changed the law again to give Patrick power to appoint a temporary successor.