Two of Vermont’s most prominent Democrats will campaign for Hillary Clinton this week, sending a clear message to fellow Vermonter Bernie Sanders as he gains on Clinton in the race for the Democratic nomination but struggles to win support from Democratic leaders.

Gov. Peter Shumlin will appear with Clinton in New Hampshire Thursday at an event on opiate addiction, though it’s not his first time campaigning for Clinton since endorsing her in May. The bigger prize for the Democratic front-runner will come Saturday, when former Gov. Howard Dean will speak on Clinton’s behalf to 3,000 Massachusetts Democrats at the state party’s annual convention in Springfield.

Dean endorsed Clinton in December, snubbing Sanders, who is currently mounting an insurgent presidential bid similar to one Dean ran 11 years ago. And Dean appears to be stepping up his work for the former secretary of state’s campaign as the threat from Sanders becomes more real.

In late August, Dean traveled to New Hampshire to campaign for Clinton, Clinton herself will speak at the New Hampshire Democratic Party’s convention, along with Sanders and other candidates.

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While Dean and Sanders have often been compared, and share much in common, the two have never had a particularly warm relationship, according to people who know them both in Vermont. Sanders served as Vermont’s only representative in Congress at the same time Dean served as the state’s governor, but Dean was more moderate as a governor than as a presidential candidate and the two had different politics, personal styles and constituencies.

In his 1997 book “Outside in the House,” Sanders called Dean “a moderate-to-conservative Democrat,” which might as well be a four letter word for Sanders. When Sanders ran for reelection to Congress as a independent in 1996, most of the Democratic Party, including President Bill Clinton, tacitly supported Sanders over his Democratic challenger, Jack Long. “To the best of my knowledge, Governor Dean is the only major Democrat to come out for [Long],” Sanders noted at the time.

The former governor’s presidential bid made him a liberal icon not unlike Sanders, so he’s a valuable surrogate for Clinton as he can vouch for her progressivism and defend her from the left. And his star power makes him a desirable guest for party events.

But Dean’s support for Clinton has separated him from many of his former supporters, who have lined up behind Sanders and previously tried to draft Sen. Elizabeth Warren into the 2016 presidential race. In fact, Dean ended up at odds with his own brother, who runs the group that spun off of Dean’s presidential campaign, when that group worked to draft Warren.

And the former governor and Democratic National Committee chair’s work for Clinton underscores perhaps Sanders’ biggest challenge as he seeks to win the Democratic nomination. Despite a steady climb in polls, Sanders still faces a deep doubts about his electability, which could deter some Democrats from ultimately pulling the lever for him.

A new CBS News/New York Times poll out Wednesday found that Sanders’ support among Democrats nationally has risen 10 percentage points since late July, while Clinton’s has fallen by a similar amount. Clinton now leads Sanders 47-27%. That’s a comfortable 20-point lead, but half of the 41-point lead she had a month ago.

But when asked which candidate had the best chance of winning the general election, the likely Democratic voters surveyed overwhelmingly picked Clinton: 55% said Clinton was best positioned to win, compared to just 10% who said Sanders. Clinton’s numbers have fallen and Sanders’ have risen, but they started at opposite ends of the spectrum (78% and 5%, respectively) so the gap remains very wide.

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As NBC News’ Mark Murray noted this week, political scientists have found that endorsements are the most important predictor of success in a primary campaign. So far, Clinton has wracked up dozens, while Sanders has failed to secure a single sitting congressman, senator or governor, according to FiveThirtyEight’s endorsement tracker.

Even politicians who know Sanders best, like Dean and current Vermont Gov. Peter Shumlin, have sided with Clinton. Vermont Rep. Peter Welch has yet to endorse and would be a plum prize for either candidate.

Sanders himself has acknowledged that this is one of his biggest challenges in convincing Democratic leaders to support him.

“Look, at the end of the day, people will say to me, we like you, we agree with a lot of your ideas, but can you win?” he told MSNBC during a press conference at a recent meeting of the Democratic National Committee in Minneapolis. “The issue of electability – can we win this election? Can we defeat the Republicans?”

Sanders went on to say that he’s convinced he can win, but he’s yet to convince the rest of the party.