From 1999 through 2013 Netanyahu's favorable score hovered around 34-35% and his unfavorable has also been flat between 20-23% .
As one would expect, the growth in support for Netanyahu is slowest amongst Democrats who have been the leaders in attacking Netanyahu.
In fact just as many Democrats have an unfavorable view of Netanyahu as have a favorable view, but Republicans and independents are much more favorable toward Bibi.
For some reason the Democratic Party has always been seen as slightly more pro-Israel than the GOP, the truth is that both parties have always been pretty equal in their support. However as the Democratic party has moved from center-left to radically progressive their support of Israel has waned. On the other hand as the Netanyahu has become a leader in the fight against terrorism and America's president has lead a retreat, Republicans and Independents have gravitated toward supporting the Israeli leader.