Buzz is growing about Sen. Elizabeth Warren’s interest in joining former Vice President Joe Biden atop the Democratic presidential ticket, but one of the biggest obstacles she faces is the possibility that her Senate seat could wind up getting filled by a Republican.

Warren (D-Mass.) is in regular contact with Biden, the presumptive Democratic presidential nominee, and plans to hold a big online fundraiser for him on June 15. The high-profile progressive has also increased her outreach to Biden’s longtime Senate allies, sending the message that she’s eager to join Team Biden, according to Senate sources.

Picking Warren could give Biden a boost in the polls. A Morning Consult–Politico survey released Wednesday found that 26 percent of registered voters said they would be more likely to vote for Biden if he chose Warren, putting her ahead of the other eight women he is said to be considering.

Her greatest impact, according to the survey, would be among voters under the age of 45, as well as black and Hispanic voters — three blocs Democrats see as key to winning the White House and picking up Senate seats.

But picking Warren would likely cost Democrats one of those Senate seats, at least temporarily, if Biden were to defeat President Trump.

Warren represents a state with a Republican governor, Charlie Baker, who has the power to pick anyone he wants to fill her seat, should it become vacant, until a special election sometime in 2021.

Massachusetts law requires a special election to be held between 145 and 160 days after a Senate seat becomes vacant, a significant problem if Biden wins the White House and the Senate is evenly divided between Republicans and Democrats.

If Baker were to appoint a Republican to replace Warren, Democrats would need a net pickup of four Senate seats — instead of three — and control of the White House to win back the Senate majority, where the GOP holds a 53-47 advantage.

Meanwhile, two other potential VP candidates — Sens. Kamala Harris (D-Calif.) and Amy Klobuchar (D-Minn.) — represent states with Democratic governors, meaning they don't come with the same risk of a GOP successor.

Given how the Senate battleground map is shaping up, a 50-50 Senate tie — counting both Massachusetts Senate seats in the Democratic column — is becoming a distinct possibility.

How quickly a potential Baker appointee could be casting votes in the chamber would be determined by Warren.

If she were to resign on Nov. 4, the day after Election Day, that could shave off 61 days during which a temporary appointee would serve in the 117th Congress, assuming the Senate convenes on Jan. 4, the first Monday in January, as it often has.

That would still give a Republican senator at least 84 days to serve in a new Senate session.

If one seat means the difference between three more months of GOP control, that could seriously blunt the first 100 days of a Biden administration, potentially delaying the confirmation of Cabinet nominees and the consideration of Democratic-backed legislation.

Some strategists say it’s a gamble worth taking.

“To me it’s become increasingly clear that she would be the strongest pick electorally,” said Mike Lux, a Democratic strategist who worked in the Clinton White House. “She’s actually very strong with African American and Latino and Latina voters.”

Lux said Warren “also really increases the enthusiasm level, which is one of the key things the Biden folks need to figure out.”

“There’s this big enthusiasm gap in a lot of the polls between Biden and Trump, and I think Warren would really help change that dynamic,” he added.

Warren herself appears enthusiastic about the VP possibility.

One Senate Democrat said her jockeying for the veep slot has been noticeable, while other Senate sources noted she’s been reaching out more to Biden’s allies on Capitol Hill.

This month she co-sponsored Sen. Christopher Coons’s (D-Del.) Pandemic Response and Opportunity Through National Service Act. The legislation would expand existing national service networks such as AmeriCorps to help respond to the coronavirus crisis.

She also co-sponsored a bill led by Sen. Bob Casey Jr. (D-Pa.), another Biden ally, to better protect the elderly and people with disabilities living in nursing homes, intermediate care facilities and psychiatric hospitals from the pandemic.

Biden and Warren have been speaking regularly in recent weeks, about once a week, according to sources with knowledge of the phone calls.

Warren has also spoken to former President Obama, mostly about her ideas surrounding the pandemic. 

In recent months, Obama has told those around him that he was impressed with the ideas she had on the campaign. Last fall, he went to bat for her when speaking to donors who were turned off by her politics, vouching for her political chops and her fluency on policy.

"With the economy in the gutter, I think he believes she can play a big role in the recovery," said one Obama ally. "He appreciates her ideas."

One Biden ally said the former vice president also "appreciates and respects" Warren's ideas.

"She's at the top of the list for a reason," the ally said.

Liberal strategists who want to see Warren on the ticket say the overwhelmingly Democratic state Senate and House in Massachusetts could change the state’s law to require Baker to pick someone from the same party as Warren.

Democrats control 34 seats in the Massachusetts state Senate while Republicans hold four, with two vacancies. In the state House, Democrats hold a 126-31 advantage, with seats vacant.

Six states require the governor to appoint a replacement of the same party as the departing senator, according to a 2017 report by the National Conference of State Legislatures.

Lux said Baker’s potential appointment of a Republican successor “is pretty easy to get around given that Democrats have a veto-proof majority and could easily change the law to make it more like other states where if the person who leaves is a Democrat than they have to pick a Democrat.”

Debra O’Malley, spokeswoman for the Massachusetts secretary of the commonwealth, said, “There’s no provision for the state legislature to veto the choice by the governor.”

But O’Malley noted that the Massachusetts state legislature could change the law between now and next year and override a veto.

“This is the law as it stands now. That law could change and in fact that law has changed multiple times in the last 20 years,” she added. 

Charles Chamberlain, chairman of liberal advocacy group Democracy for America, said it would be worth changing Massachusetts election law to get Warren on the ticket. 

“If she were to join the ticket as the vice presidential candidate, I think it would make progressives across this country very excited for this ticket,” he said. “It would really make a difference in motivating, energizing and engaging progressive voters.”

Chamberlain said even if Baker picks a temporary Republican successor, that could pave the way for Rep. Ayanna Pressley (D-Mass.) to make the leap from House to Senate.

“We’re talking about a very short period of time until the next special election, when we could be looking at Ayanna Pressley as the next senator from Massachusetts.”