The best way is to see a few campaign ads. Senate Majority PAC and Americans for Prosperity best show the two sides (Dems and GOP, respectively)
http://www.youtube.com/user/majoritypachttp://www.youtube.com/channel/UC-JY5LuDM2wzN8cd5ubvRsgGOP = Obamacare failures(including Medicare cuts from ACA), Keystone pipeline, deficits, balanced budget amendment, gas prices, weak economy, war on coal, immigration
Dems = Obamacare successes, equal pay for equal work, abortion and Planned Parenthood, Violence Against Women Act, disaster aid, farm bill, Medicare/social security privatization (opposition to it), minimum wage, higher taxes on wealthy, Koch Brothers = evil, education
Republicans tend to campaign the same way in every state with fewer differences. Democrats tend to focus on each race a little differently.
Border control is a minor, minor issue right now, but it might be emphasized more later. Obamacare is the GOP's main theme.
Foreign crises aren't really being used that much in campaigns at this point; ditto with Benghazi.
Most Americans hate Congress, but they blame the other party for all the problems, in general.
Economic growth is too negligible to affect the midterms. Look at 1994. However, the Dems would be further in the hole with a bad economy.
The lawsuit is just pure polarizing: Republicans love it, Democrats hate it, independents are generally opposed to it but I'm sure a lot agree with it too.