That's quite a precarious limb right there. If Ted Kennedy Jr., a freshman state senator, somehow becomes the nominee, Democrats deserve to and will most certaintly lose.
He's not any freshman state senator, though. He's a Kennedy. His name recognition exceeds that of Lembo, who has won on the statewide level, and he brings a significant amount of wealth to the campaign. His name recognition will also likely allow him to advance his legislative agenda at an accelerated rate compared to other senators. In 2018, he'll have already won reelection.
