Chuck Grassley has been a Republican Senator from Iowa since 1981. In that time, he has won every re-election campaign by huge margins - better than 2:1 until 2010 and that year he got 64% of the vote.
Yet, recent polling shows him with a very narrow lead over Democratic candidate Patty Judge. http://www.realclearpolitics.com/epolls/2016/senate/ia/iowa_senate_grassley_vs_judge-5978.html
A Democratic win in Iowa would go a long way in locking up a Democratic Senate. Will it happen? Why is Grassley looking like he's going to have his first competitive election since 1980?