Tennessee's been getting rapidly politically polarised toward the Republican direction over the past decade as the moderate Democrats die out or change affiliation to Republican. I guarantee Harold Ford Jr. would have lost the next election, and that goes double if Obama still wins in 2008.
There's also the popular perception in Tennessee that he was a scumbag since he came from the Ford family of Memphis who have deep, deep links to local and state corruption. Case in point, his Uncle John Ford was nailed in 2005 for multiple charges of bribery, threatening FBI agents, etc. A couple years in the Senate will probably only increase that perception.
Ford will then likely lose in 2012. He might not even run for reelection if he's facing near-certain defeat. His political career afterwards is probably just as stalled as it is currently.
Basically. The statewide Democratic Party will probably still back him regardless, but he'd be looking at a landslide loss to whoever the Republicans dig up. It'll probably be a Tea Party guy too who wins the primary, so maybe Joe Carr.
To see how little effort the Tennessee Democratic Party puts into important state elections, look no further than who they "nominated" in the 2012 US Senate election (far-right anti-gay activist) and 2014 Governor election (random old guy from the middle of nowhere named Charlie Brown). I suspect that's because any serious Democrat will focus on other state or local offices.