Clinton's Poll Numbers Rival Obama's in 2008, Significantly Better than 2012
I'm going to base this thesis on the RCP averages, despite them having a tendency to screw Republican and underestimate the Democrats, mostly just because RCP is the only averages that I can find from the last 8 years.
Currently, Clinton holds a 4.6 point lead in the RCP averages and closer to 5.5-6.0 points in other averages. On July 5th, 2008, Obama had a 5.7 point lead over McCain before dropping to around 4.5 points a few days later. During the entire 2008 election up until July 5th, McCain lead Obama for 16 days in the RCP averages. So between September 1st, 2007 and July 5th, 2008, Obama lead McCain for all but 2 weeks. McCain's largest average lead over Obama was 2.7 points in January 2008. Thus far in the 2016 election, Trump has lead Clinton just 4 days with his maximum average at just 0.2 point lead. Keep in mind that McCain lead Obama 16 days over a 10 month period. Trump has lead Clinton just 4 days over a 13 month period. Clinton's largest lead over Trump came 1 year ago, where she lead him by over 19 points in the RCP average, far greater than Obama's largest lead at this point, of 7.1 points in June 2008. However, if we just look at the actual calendar year of 2016, Clinton still blows Obama out of the water with an 11.2 point lead back in late March.
2012 was a very different story than 2008. By July 5th, 2012, Romney had not been the leader in the averages for a single day, however, the averages were significantly closer. Polls had been conducted for 9 months by this point in 2012. Obama's largest lead by July 5th, 2012 was 6.1 points over Romney which came in late February 2012. On July 5th, 2012, Obama was up by just 2.6 points, which is just over half the lead that Clinton currently has over Trump. For most of the spring and summer of 2012, Obama's lead over Romney was hovering around 2 points and in 2008, his lead over McCain was 5 points. For much of June and July, Clinton's lead over Trump has hovered around 6 points.
Based on the RCP averages, Clinton is doing significantly better than Obama was in the 2012 campaign and is matching if not slightly beating where Obama was in the 2008 campaign against Trump.
I didn't post any of the graphs in this thread because they are truncated and thus its difficult to visually compare year to year. For example, Obama's 6 point lead in 2012 visually looks at large as Clinton's 20 point lead in 2015 because RCP scales the graphs based on the min and max leads
Links:
http://www.realclearpolitics.com/epolls/2008/president/us/general_election_mccain_vs_obama-225.html#chart
http://www.realclearpolitics.com/epolls/2012/president/us/general_election_romney_vs_obama-1171.html
http://www.realclearpolitics.com/epolls/2016/president/us/general_election_trump_vs_clinton-5491.html