It's probably a 5 year balloon, amortized over 30 with low payments. Which means he'll have to refinance or sell at the end of 5 years unless he's got half a million bucks laying around. Assuming his credit is OK, refinancing wouldn't be a big deal at that point. Even if he's not that wealthy, his Senatorial salary should cover the payments as long as he doesn't have other major debts.
Of course, if he loses in November...then who knows...
Actually, when a senator loses an election, they usually get a private sector job that pays about five times what their senate salary was.