"And it's clear now that the outcome of these and other fights will probably rest on one vote in the United States Senate," he said.
National Democrats are shocked that the race seems this close, given that Democrats outnumber Republicans in Massachusetts 3-1. Some say it is less about the president and more about a weak candidate. Although Coakley made her name as a hard-nosed prosecutor in the famed Louise Woodward nanny trial in the 1990s, some Democrats say she has run a flat , flawed campaign. For example, last week she called former Red Sox baseball player and hometown hero Curt Schilling a Yankees fan, for supporting Brown.
But some Republicans say the mere closeness of the race spells troubles for Democrats.
"Whether Coakley wins or loses, the message is the same, health care. This health care package is death for candidates," said conservative commentator Tucker Carlson.
ABC News' Jake Tapper, Jonathan Karl and Kristina Wong contributed to this report.