Mark Kirk (R-IL) becomes the newest senator today when he's sworn in by Vice President Biden (D) at 5:30 p.m. ET.
Politico: "His arrival will narrow Democrats' majority in the Senate to 58-42, meaning they'll now need at least two Republicans to reach the 60-vote threshold to move forward on legislation, from a limited extension of Bush-era tax cuts to a repeal of the Pentagon's 'don't ask, don't tell' policy."