It could also force Obama to veto a lot of legislation.
Which 9 Democrats are going to pass legislation Obama would be forced to veto?
Remember this Senate (probably) no longer has Baucus, Begich, Landrieu, Hagan, Pryor, etc.
Presumably, the 9 most conservative Dems still around would be:
Manchin, Donnelly, McCaskill, Heitkamp, Tester, Warner, King, Casey, and Heinrich.
I think a President Obama in his last two years, desperate to do anything really, is going to go ahead and pass any law Martin Heinrich approves.
If Majority Leader McConnell nuked the filibuster on legislation, I would blow the guy in celebration.
Reid has weakened the filibuster, so that alters discussions of necessary majorities.
With control of the Senate, Republican would also have the advantage of choosing what legislation is brought to the floor. That would force some Democrats in awkward positions when it comes to items with broad support.
It is worth noting that we don't know how man Republicans will be in the Senate in 2015. King has said that he might caucus/ conference with whatever party holds a majority.
If Republicans run the table, they'll end up with a 56 seat majority.
It doesn't guarantee a GOP win, but it does help in a few ways.
First, it's better for the Democrats if the party's reputation is stronger.
Obama is also more restricted with a Republican Senate which limits his ability to get the accomplishments that would raise his approval rating.
It could also force Obama to veto a lot of legislation. If Republicans are smart, they'll force him to veto legislation with broad support. Of course, Republicans can also overreach, which would result in Obama gleefully vetoing unpopular legislation that passed through party line votes. And base disappointment with a Republican led to Congress could also lead to the party nominating someone far-right.
If Republicans win Senate elections in every state Romney won, this would result in the party getting a majority in the Senate. But these states wouldn't be enough to swing a presidential election. Even wins in swing states (which Romney tended to lose) wouldn't demonstrate that the same electorate will show up in a presidential election.
You love to cite the post-WWII history of parties running for 3rd consecutive terms being handicapped. However, the post-WWII history on congressional and presidential elections suggests just as strongly that an opposition takeover of congress in a midterm helps the incumbent president's party win the next presidential election.
The examples are: 1946-48, 1954-56, 1986-88, 1994-96, 2006-08 and 2010-12. We could also suggest that 1982 constitutes an opposition takeover of the House based on conservative Democrats voting with Republicans to pass Reagan's agenda in 1980-82 (although this calls into question whether a liberal Democratic majority operated in 1955-56). So in 5/6 or 6/7 recent cases, the next presidential election was a robust incumbent party win.
I don't think the figures suggest that losing midterm elections help a party keep the White House.
It is a small sample set consisting of six elections, four involving Presidents who would run as incumbents.
Three of the elections you mentioned were in the first term after a President took the White House for his party. In those cases, Presidents tend to get reelected. There's only been one President who failed to keep the White House for his party for more than a term in over a century.
Truman may also be an incumbent, but he was able to run against a do-nothing Congress, so that would be a precedent that makes Hillary Clinton or whoever Democrats nominate happy.
Only one of the midterms you mentioned was followed by an open presidential election in which the incumbent party kept the White House. It also involved Papa Bush losing over ten percent of Reagan's vote. For him, it was enough to keep the White House for Republicans. That's unlikely to be true of the next Democratic nominee.
I'm not suggesting that Democrats are doomed, and I've listed a few ways the GOP could shoot themselves in the foot with control of the Senate. But I think it would certainly be wrong to interpret a Republican win as a good thing for Democrats.
