We may be at such an historic moment, but just for today I don't know that normal political calculus would point to this shift happening. The levers of power that dominate the government in Mississippi - as well as poll stations and ballot boxes - are so heavily stacked towards keeping the friends of the powerful going to Congress that it's doubtful that they'd lift one finger to help a Democrat take a Senate seat away from them, no matter how noxious Cochran's courting of blacks and other Democrat voters may be to them. From that perspective, we may see a November campaign in Mississippi reminiscent of Joe Lieberman's Senate run in 2006, in which he was forced to run as an independent candidate in the general election after Ned Lamont surprised the Connecticut Democratic establishment with an outsider-style primary campaign that seized the Democratic nomination from Lieberman. Both mainstream Democrats and, quietly, key Republicans fueled Lieberman's re-election in the 2006 general election to keep the status quo power train intact in the Senate. Cochran managed to keep his party's nomination by comparison to Lieberman, but he will have to fight the Tea Party uprising as hard as Lieberman fought off Lamont's insurgency.
And in fighting off the Tea Party, it's highly probable that Cochran is not going to be able to throw away the alliances with blacks and moderate Democrats that fueled his primary win. Just as Lieberman swung hard to the right (not hard for him) to woo Republicans to cross over to vote for him, so will Cochran rely heavily on Democrats to make the difference in the general election. In doing so, Cochran will be obliged to that new cadre of swing voters for his future political survival - just as Stennis and other Democratic senators from the South had to learn how to woo Republican voters as the demographics of party identification shifted in the wake of the civil rights legislation of the 1960s.
In other words, what we may be seeing is the emergence of the mirror image of the "blue dog Democrats" who managed to hold on to "D" seats in Congress by finessing Republican and independent voters on social issues that mattered to them. Now "R" seats may be beholden to Democrats who will force Republicans to be more responsive on social and economic issues that matter to them - call them "red dog Republicans" if you will. Not Republicans in Name Only, really, but Republicans who know that they are cooked if they cannot reach out to a block of voters that will allow them to overcome challenges from the far right.
Is it better to just go for a straight Democratic win in November for Mississippi's Senate seat? If the balance of power in the Senate is not at issue, I'd say that it's maybe more beneficial to make Cochran a red dog Republican. It means that he will have to vote with Democrats often enough to keep his new coalition happy - meaning that he'll have some control over the substance of legislation, but probably far less than he would if he were a blue dog Democrat. In other words, maybe it's better to allow the normal flow of political change to take its course - and reintroduce the concept of Senators having to respond to a broad constituency in order to be re-elected. I don't know if that happens if we get a Democrat elected to the Senate - that's more likely to turn into a "blue dog" scenario, which has been a real stumbling block for forming hard-hitting legislation in the Senate.
We'll see how this turns out. All things being equal a straight-out Democratic win in the South for the Senate would be an awesome thing in its own way, but I am not discounting the potential beauty of a red dog Republican forcing other Republicans to learn some moderation - out of political necessity.