Nicole Gaudiano

USA TODAY

Sen. Patrick Leahy, under pressure in Vermont to switch his support from Hillary Clinton to Sen. Bernie Sanders in the Democratic presidential race, said Tuesday he'll cast his superdelegate ballot at the party's convention for the candidate with the most pledged delegates.

“I will be at the convention and ... I fully expect to support our nominee,” Leahy said during an interview at the Capitol. “I’m a lifelong Vermonter. The biggest part of my reputation is keeping my word. I don’t break my word. But I will vote for the … candidate who has the most pledged delegates, and I assume well before the convention we’ll know who that is.”

Nearly 5,000 people have signed an open letter posted online by the advocacy group Rights & Democracy urging Leahy, Vermont Gov. Peter Shumlin, former governor and former Democratic National Committee Chairman Howard Dean and party Committeewoman Billi Gosh — all Clinton supporters — to cast their superdelegate votes for Sanders at the convention.

Sanders' supporters say he deserves backing from Vermont's superdelegates because he won 86.1% of the vote in the state's March 1 Democratic primary. A fifth Vermont superdelegate, Democratic Rep. Peter Welch, already has endorsed Sanders.

Growing backlash demands superdelegates vote Sanders

Leahy pledged his support to Clinton during a trip with her to Haiti in 2012, when she was still secretary of state. That was long before she or Sanders announced their presidential campaigns, according to a November statement from his office.

He told the Burlington Free Press in February, before the New Hampshire primary, that he would back Clinton and said casting his superdelegate vote for her was a matter of “conscience and commitment.”

This week, Leahy's campaign manager, Carolyn Dwyer, wrote in an email that Leahy’s personal support is with Clinton but his vote as a superdelegate will go to the candidate leading in pledged delegates going into the convention. Superdelegates are "unpledged" Democratic Party officials and leaders who may support any candidate they choose.

Clinton now has 1,289 pledged delegates and 469 superdelegates, compared with 1,038 pledged delegates and 31 superdelegates for Sanders, according to the Associated Press.

The online letter asks all Vermont superdelegates to cast their votes for Sanders.

“While some superdelegates might have made a “promise” in the past to other candidates, it would be irresponsible and undemocratic to ignore the Democratic primary results,” the letter states. “Both the current and past presidential election cycles have been marred by instances of voter suppression and other assaults on the democratic process, but today you have a unique opportunity to uphold the democratic tradition and restore faith in our electoral process.”

Contributing: Dan D'Ambrosio, Burlington Free Press

Follow @ngaudiano on Twitter.