The last diary I wrote concerned the general expectations of the Congressional elections, and since this will definitely have a substantial impact on the Congressional elections, I think we need to look at how specific presidential candidates affect the congressional races.
To keep this from getting too out of hand, I'm only really looking at the top candidates for each party (Democrats: Clinton, Obama, and Edwards; Republicans: Giuliani, Thompson, Romney, and McCain)
Disclosure: I'm supporting Barack Obama in the presidential primaries.
Democratic Candidates
Hillary Clinton: Ok, to get this out of the way, Hillary Clinton is the frontrunner and, at the moment, most likely to win the nomination. The congressional races she helps are going to be in already blue states: Oregon (Gordon Smith) New Hampshire (Sununnu) possibly Minnesota (Coleman). In House races, she again helps freshman who are in blue districts and states and will help our efforts against the remainder of the northeast Republicans (Chris Shays, the lone New England Republican for one). Now here's the bad part: she's not all that helpful in the midwest or the south where quite a few Senate races could become competitive if the conditions are right: Kentucky (McConnell) Texas (Cornyn) North Carolina (Dole) Alabama (Sessions) Colorado (open) and Tennessee (Alexander).
My analysis: Clinton wins at the cost of the other tickets. Bill Clinton did the same thing in the 1996 election and that's what I think would happen here.
Barack Obama: Alright, Obama is the number two and, depending on the circumstances, could upset Clinton. So what does Obama do for the congressional ticket? Well, he's actually a ticket enhancer. Obama's candidacy will boost turnout of black voters and young voters (blacks vote at least 90% for Democrats and in southern states like Tennessee, Kentucky, and Alabama it could lead to some good upsets). Additionally, Obama also seems to do well among independents which would definitely be good for the two freshman congressmen in New Hampshire and in some red districts where winning over independent voters is important.
My analysis: Obama is a ticket-lifter, he doesn't inherently hurt the ticket the way Clinton does and, in fact, depending on who the Republicans pick, could actually put some extra seats into play that otherwise wouldn't be competitve.
John Edwards: Ok, Edwards is the dark-horse right now, and unless something really dramatic happens he's not winning the nomination, but having said that, here's my analysis of his impact on congressional races. Edwards is definitely a popular figure in the midwest and in a few of the southern states (North and South Carolina for instance). He'd definitely help candidates like Pryor and Harkin in Arkansas and Iowa respectively, and could probably do something about Elizabeth Dole (seriously, why doesn't he just drop out of the presidential race and just rerun for the Senate against Liddy Dole?).
My analysis: Edwards is more of a help than a detriment, and, quite frankly, had he been the nominee in 2004, we'd probably have picked up a couple of those southern seats in the Senate. Edwards share Obama's ability to make some southern seats more competitive, though his being on the ticket in 2004 does make him more polarizing than in previous years and being able to be linked to Kerry in the south could do more harm than good to congressional races. (John, it's not too late to challenge Liddy…)
Republicans to be added tomorrow
If Clinton hurts us in the midwest, she almost certainly isn’t winning Ohio, without which Democrats are not winning the White House. She could also throw Minnesota, Iowa, Wisconsin, and even Michigan to the Republican if she is as polarizing as you say.