Portman could easily run for reelection while pursuing the Vice Presidency, not sure about New Hampshire or South Dakota election laws regarding running for two offices at once. If there is a prohibition against seeking two offices at once, then I can't conceive of either Ayotte or Thune running.
While Ohio state law may let him seek both the Vice Presidency and the Senate seat, it won't look good for Portman to do both. It would give Strickland a great way to attack Portman, saying Portman is more focused on his own political career than the people of Ohio. In a swing state like Ohio, Portman will need to focus all his attention on his senate seat to defeat Strickland, it is expected to be a competitive race. Same idea applies to the Ayotte / Hassan race in NH. Thune, on the other hand, is running in a safe republican state; so even if his Dem challenger accused Thune of focusing more on himself than the people of SD, it likely wouldn't matter too much.
