I think any notion of "uniting the country" has to come with the understanding that the primary electorate does not equal the general electorate, and that most Americans aren't a.) all that passionate about politics or b.) particularly ideological. Someone like Booker, at least in terms of messaging, would do extremely well with a general electorate and fare poorly with an activist base, whereas Sanders/Warren would do extremely well with the Democratic base but poorly with the general electorate.
I'll chip in and say that I think Sasse (not a Democrat, obviously) would do extremely well in a general election, particularly with people who don't vote.
Firstly he's black so that automatically makes it a challenge for him to connect to the broader electorate. Obama did it, but even he struggled. Obama spent his entire career crafting an image which made him popular with working class whites. His success in downstate Illinois in his 2004 Senate race translated into his success with the WWC nationally. Do you really think Booker will have the same image as Obama had? Booker isn't from Illinois he's from New Jersey. New Jersey is a stereotypical coastal elite type of state. It's very wealthy and urban making it rather difficult for him to have the same image as Obama. Let's also not forget that Obama was a once in a generation speaker, and that also helped him have broad national appeal.
Just because Cory Booker is "fiscally moderate" doesn't mean he automatically dips into this magical pool of "moderate" voters. Electoral politics is more complicated than a simple battle of being more left leaning or more right leaning. Cory Booker has serious image problems which would prevent him from ever being capable of being a nationally uniting candidate. He's a wealthy technocrat and most Americans can't identify with him. There are certainly ways to change that image. Donald Trump managed to break loose from it, but it took a long time of careful planning and imaging. Cory Booker could try and shake of his image problems but I don't see him doing that anytime soon.
No, Obama swept through downstate because his opponent wasn't a real Illinoisan; he established residence in Calumet City a few months before the election and was really from the East Coast. Check out the Chicago Tribune's 2004 endorsement of Obama to see just how out of touch his Republican opponent (Alan Keyes) really was with Illinois. The GOP couldn't find anyone who wanted to run against Obama. Also, since Keyes entered late, Obama had already consolidated his base in Chicagoland and had plenty of time to campaign downstate. Furthermore, downstate Illinois was a lot more swingy and more apt to split ballots (Bush still won the area) than it is now. Rod Blagojevich's corruption scandal was what (I think) really turned downstate red for good, and that didn't happen for another six years.
Also, a career in government hardly screams "crafting an image to connect with the working class". If Obama wanted to do that, he'd have been in the pipe fitters' union or the UAW.
Anyway, I'm not sure whether this changes your mind on Booker, but I thought I should at least set the record straight.
