By Moira Warburton

WASHINGTON (Reuters) - Facing uphill re-election battles in 2024, vulnerable Senate Democrats are pushing legislation that promotes "Buy America" policies, attempting to bolster their party on a brand of economic populism they hope will keep them in the majority.

The Senate Commerce Committee on Thursday advanced a bill from Democratic Senator Tammy Baldwin and Republican Senator J.D. Vance to require country of origin labeling for goods sold online, bringing them in line with brick-and-mortar goods.

On Wednesday, a Senate committee approved Democratic Senator Sherrod Brown's bill requiring all American flags bought by the U.S. government to be made in the United States, and last week the Senate passed an amendment from Baldwin to an annual defense policy bill requiring the Navy to build ships in the United States with American materials.

"There's definitely momentum," Senator Gary Peters, a Democrat from Michigan and chair of the Senate Democrats' campaign committee, citing the COVID-19 pandemic as a galvanizing force. "We had very efficient supply chains, but they weren't resilient. The way you get resilience is to make sure all those products are made in the United States."

Senators serve six-year terms, and Democrats hold a narrow majority in the chamber. Next year, Democrats face headwinds when more of their members will be defending seats than Republicans. Many of those Democrats come from states that former President Donald Trump won in 2016 or 2020, including Ohio, Wisconsin and Pennsylvania, all of which have major manufacturing industries and high union populations.

Buy America policies are "mom-and-apple pie issues with American voters" that have "virtually universal support," Scott Paul, president of the Alliance for American Manufacturing, said. "In addition to having a good jobs impact, there's also a good political impact."

Although Democrats have long been the party of labor unions, Trump's anti-China, pro-American-worker message in 2016 challenged that notion after blue-collar voters without college degrees flipped to him in droves, said Nick Iacovella, vice president at the Coalition for a Prosperous America which represents domestic producers.

In the aftermath of Trump's win, both parties are scrambling to "figure out how to become the party of the 21st century American worker," Iacovella said. "It boils down to, 'Voters think the China issue is important and we should be unapologetically pro-worker and anti-big business and anti-Wall St.'"

Such legislative priorities have long been pushed by Baldwin and Brown. Both will face tough re-election battles next year - Brown in Ohio, which Trump won with 53% of the vote in 2020, and Baldwin in Wisconsin, which voted by margins of less than 1% for Trump in 2016 and U.S. President Joe Biden in 2020.

"When we are using taxpayer dollars, we should be supporting U.S. jobs and small businesses," Baldwin said. "Our previous president talked a lot about it, but wasn't successful in getting those provisions through."

Buy America bills often run in to opposition from corporate-minded lawmakers and pro-business associations.

John Murphy, senior vice president for international policy at the U.S. Chamber of Commerce, said "Buy America" policies have "serious limitations" and impose rising costs for businesses without increasing production.

But the tide among Democrats is turning against these arguments.

"People are seeing the mistakes we made as a country on China trade over the years," Brown told Reuters. "Too many Democratic - really presidents in both parties, from Bush to Trump - were complicit. We lost far too many jobs."

(Reporting by Moira Warburton in Washington; Editing by Matthew Lewis)