Biden also slammed Congress during her remarks, blaming the administration's stalled agenda on Republicans. Joe Biden's sweeping Build Back Better plan -- which would have expanded the nation's social safety net -- was served its latest blow this week when West Virginia Sen. Joe Manchin, a moderate Democrat, dismissed including any climate or tax provisions in the bill. In a narrowly divided Senate, Democrats needed Manchin's support to pass the legislation along party lines in a process called budget reconciliation, which requires all 50 members of the Democratic caucus to agree to advance legislation.

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"I know there are so many nay-sayers who say we'll get slammed in the midterms. Okay. The Republicans are working hard, they stick together, for good or evil. So, we just have to work harder," she said.

Saturday's event marked the second DNC fundraiser the first lady has attended during a two-day swing to Massachusetts. On Thursday, she made remarks, predominantly focused on political action, at a private event in Andover.

CNN's Jasmine Wright contributed to this report.