Maryland Sen. Barbara Mikulski (D) becomes the longest-serving female senator in history as the 112th Congress begins, former House Speaker Nancy Pelosi (D-Calif.) tweeted.

Mikulski has served in the senate since 1986 and won reelection to a sixth term last November.

"112th Congress is about to begin - watch live at http://houselive.gov - as @SenatorBarb makes history, longest serving woman!" Pelosi tweeted.

Mikulski is one of 12 Democratic women currently serving in the Senate.