The Republican party is headed towards a very difficult election year. It wouldn't surprise me if they ended up losing about 25 more seats in the House and 4-5 in the Senate. The Democrats could do better, but they haven't exactly done a good job in managing the majority they have now. What is proping them up is the fact that people like Bush even less.
On the Presidency, I believe that McCain odds of beating the Democrats are about even. Surprising considering the bad economy and the War in Iraq, but McCain is actually seen as an agent of change and that benefits him. He has been critical of Bush's handling of Iraq and on other issues, he diverts from his positions as well, like on global warming and torture. All of that has helped the Republicans even out the field. It also helps that either Hillary or Obama are not the strongest candidates. The Republicans may not love McCain, but he gives them their best chance. It will be a close race for the Presidency, but I am fairly certain that the Democrats will build on their majority.