What would change if the Republicans take the Senate? Democrats have gotten all of their obstruction on the sly up to this point. Harry Reid can just not vote on over 300 bills the House passed and the Pravda Press can ignore all of them. That will change if the Republicans take the Senate. Filibuster votes are on the record. They become issues in subsequent elections. Also, remember the reconcilement bill rule that Democrats used to pass Obamacare with a simple majority. Budgetary savings can be passed easily. Then our Dear Leader will have to veto spending bills. Suddenly the optics of the "party of no" change a lot. Democrats are the obstructionists filibustering and vetoing everything. Republicans are trying to govern. The Pravda Press has to cover the issues involved in the filibusters and vetoes instead of ignoring them. Suddenly there's a big political climate change.
The problem for Republicans and Democrats is different. Republicans brag about their obstruction of Obamacare because it's so unpopular with voters. The split in the Republican Party is mainly over tactics, not substance. Should we hold out for more and filibuster, or take the deal on the table? I think the reason that the shutdown did no long term damage to Republicans was that it was about postponing Obamacare and subsequent events showed that Obamacare should have been postponed. The Democrats, on the other hand, vote liberal in DC and talk "centrist" at home. Votes on the record against Keystone Pipeline would be very unpopular with organized labor while pleasing environmentalists. These votes would serve as wedge issues, splitting the Democrat's base. Harry Reid has allowed the Democrats to have it both ways on Keystone and similar issues. If there's Republican Senate, the free ride is over and the wedge issues begin to grab headlines.