Her entreaty came as her husband, President Barack Obama, capped a three-day fundraising swing on the West Coast, bringing in millions of dollars for the committees charged with electing Democrats to office.

“We need you to write the biggest, fattest check that you can possibly write,” Mrs. Obama told a crowd of donors in Chicago, her hometown. “Writing those checks is the single-most impactful thing that you can do right now.”

The president and first lady, along with Vice President Joe Biden, have stepped up their fundraising schedule in recent weeks as November’s congressional contests approach. Democrats’ chances of taking control of the House appear slim, and Republicans badly want to pick up the six Senate seats they’ll need to take over that chamber.