In a primary contest that's tested the populism of Democratic presidential hopefuls, Bernie Sanders could get a boost from some members in one of the country's most powerful labor unions.

A small but vocal number of Service Employees International Union (SEIU) members are signing a petition to prevent their International Executive Board from announcing an early presidential endorsement, particularly one that favors Democratic frontrunner Hillary Clinton.

"We are supporters of Senator Sanders and believe his voice deserves to be heard," the petition reads reads. "His campaign is drawing thousands into a movement around the very issues we support in our day-to-day organizing. To make an early endorsement of Hillary Clinton would put our union in direct opposition to this growing movement."

Rand Wilson, a communications director for Local Union 888 who moonlights as a volunteer for the "Labor for Bernie" campaign, added that "it's just premature" to be making a decision without debate.

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"Hopefully, the momentum grows internally, and that the members of the International Executive Board that will be giving support for a particular candidate will pay heed," Wilson told CBS News. "We want them to listen to what people are saying at the grassroots."

According to Wilson, about 300 SEIU members, retirees, and staff have already signed the petition. They're aiming to pass the signatures on to union officials a few days before the board announces their endorsement.

The petition comes as a new poll shows Sanders catching up to Hillary Clinton in Iowa, coming within seven points of the Democratic frontrunner.

Labor union members who support the "Labor for Bernie" movement say that Sanders, who is for a $15 minimum wage and has made income inequality a central point of his 2016 campaign, aligns closely with the organization's values. SEIU heads the "Fight for 15" campaign, which has focused on raising wages for low-level workers. It also supports a pathway to citizenship for immigrant workers.

The labor union, with over 1.8 million members and a robust political spending operations, has considerable sway in the Democratic primary race.

In 2008, the SEIU endorsed then-candidate Barack Obama over Clinton, though the union gave wiggle room for local unions to give their own endorsements by state.

This year, Clinton's campaign has made significant strides to secure the union's support. In June, she endorsed the SEIU's campaign to to raise fast food workers' wages to $15 an hour.

"No man or woman who works hard to feed America's families should have to be on food stamps to feed your own families," Clinton told an audience of low-wage workers in Detroit. "It is wrong that so many people stand against you thinking that they can steal your wages with no consequences. That even stacks the deck higher for those at the top."

Clinton also met with child care workers earlier this month during an event in Los Angeles put on by SEIU.

SEIU officials have said that the majority of union members have shown support for Hillary Clinton. According to Politico, SEIU leaders say that internal polls have 75 percent of members showing favorable approval of Clinton.