DES MOINES, Iowa — Nine outside groups backing Bernie Sanders will begin coordinating efforts to boost the presidential candidate in what they say is the largest independent mobilizing drive in the Democratic field, aides with the coalition told NBC News.

The independent expenditure, dubbed "People Power for Bernie," will be barred from coordinating with Sanders' campaign, but members of the coalition will be able to team up on grassroots organizing and potentially advertising.

The reinforcements for Sanders comes just ahead of Monday’s first-in-the-nation caucuses in Iowa, where Sanders' surge into the lead in most polls has worried some moderates and prompted an almost $700,000 ad campaign from a pro-Israel Democratic group arguing Sanders can't beat President Donald Trump.

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"This is the political revolution in action," said Natalia Salgado, political director at the Center for Popular Democracy Action, which is part of the coalition. "Black, brown and working people are the core of Sanders supporters. Together, this powerful network of organizations will ensure that Sanders becomes our nominee."

The groups include Democratic Socialists of America, the youth climate advocacy group Sunrise Movement, and Our Revolution, which spun off of Sanders' 2016 campaign, along with the progressive organizing groups Center for Popular Democracy Action, Make the Road, Action People's Action, Student Action and Progressive Democrats of America. Also part of the effort is the criminal justice group Dream Defenders.

The groups aim to mobilize an additional 1.4 million voters for Sanders in the primaries, starting in Iowa but continuing on through the rest of the primary season and into the general election if he is the Democratic Party's nominee.

The coalition will allow the groups to coordinate their spending and pool resources, but the budget for the effort remains to be determined.

"Our movement is mobilizing to turn young people out in record numbers to nominate Sanders, defeat Trump, and back up Bernie as he fights for Green New Deal from the White House," said Stephen O'Hanlon, the spokesperson for the Sunrise Movement.

Sanders' support from outside groups has been controversial, since the candidate often rails against super PACs and so-called dark money nonprofit groups influencing politics. Critics, including some rival campaigns, grumble that Sanders has benefited from such groups, especially as Our Revolution, which he helped found and which was run by his campaign co-chair until recently.

The campaign finance watchdog group Common Cause filed a complaint with the Federal Election Commission earlier this month alleging the group exceeded campaign contribution limits. Our Revolution denied that charge and called it "meritless."

But the coalition of groups in People Power for Bernie, which are 501(c)4 nonprofits, say they represent the voices of grassroots Americans against wealthy special interests.

"Bernie Sanders' campaign is about more than one election. It's about changing politics and shifting power from corporations and billionaires to the working class," said Megan Svoboda, a member of the Democratic Socialists of America national political committee and chair of its "Bernie Campaign." "But Bernie cannot do this alone. To win this election and the demands that he’s running on, it will take a movement."