The Clinton family campaign force is expanding its numbers with a second grandchild: Chelsea Clinton announced that she and husband Marc Mezvinsky are expecting baby No. 2 via Twitter on Monday.

"Next summer, Charlotte is going to be a big sister! Feeling very blessed & grateful this holiday season," she wrote, with a picture of her 15-month-old daughter holding a Big Sisters Are the Best book.

Chelsea Clinton, 35, serves as the vice chair of the Clinton Foundation. She's also made campaign stops with her mother Hillary, who is running for president in 2016.

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Both Hillary and Bill Clinton took a moment to say congratulations to their daughter on the social network as well.

"Your dad and I could not be happier for you, Marc, and Charlotte. We're so excited to meet our second grandchild!," Hillary wrote.

"Christmas comes early! @HillaryClinton and I are thrilled for Chelsea, Marc and Charlotte's growing family in 2016!" added grandpa Bill.