Mid-term elections strongly influence the rest of the president's term in office. If his party is in the majority in the House and Senate, then his agenda will breeze through. If not, then he will be a lame duck president very early.
Every president is a lame duck following the elections at the end of his term (if he has reached the term limit or did not win re-election), but some presidents are functionally lame ducks much sooner. That's what the Republicans are hoping for this year.
George W. Bush felt this pain when his last two years were marred by a Democratic-controlled House and Senate. He has no choice but to sign spending bills that they wanted. He bore the brunt of the criticism for that, too, when people forget that presidents don't write the bills.
I see this, though, as our system in action. I think the worst situation is when one party has all the power in the House, Senate and presidency. Then there is no balance, no disagreeing voice that can stop a runaway agenda. On the other hand, it gets carried away when either side decides to use it to punish the other party.
Interestingly, a mid-term election in the first two years of the first term of a president are a vote on how well that president is doing because, again, the president bears the brunt of the government action, whether his party is in charge or not. He is the figurehead, the scapegoat at times. Like it or not, that's part of the job, and a wise president doesn't bother to whine about it--he just accepts it.
Presidents know this, and that's why they campaign hard for their party. But the president's party may still lose, which happened to George W. Bush in the last mid-term election of his presidency when Republicans lost 30 House seats and 6 Senate seats. It also happened to Bill Clinton, but much, much worse. The Democrats lost 52 House seats and 8 Senate seats in his first mid-term elections. Clinton did better in his second term, with his party gaining 5 House seats and no change in the Senate.
When we consider how poorly Democrats did in the first Clinton mid-year elections, it seems surprising that he won re-election. But it just goes to show that presidents can survive bad mid-term elections. It happened to Franklin Roosevelt, Harry Truman, Dwight Eisenhower and Ronald Reagan, too. Remember, they all were re-elected. So Republicans shouldn't rest on any results of the upcoming mid-term elections; there still is a battle to be fought in two more years.