As an independent voter, I'm hardly a fan of the Republicans whom I view as big spending, warmongering socialists and corporatists. The Senate exception is Rand Paul who is the only constitutionalist out of a body of 100 and while some Republicans are not too horrid, most are worthless sellouts.
However, there are indeed critically important situation wherein voting R for the Senate makes a whole lot of sense EVEN if the Republican has a nasty record on a lot of issues. Furthermore, it's highly unlikely that a Democrat would be an improvement over the worst Republican.
Let's examine 3 big reasons why voting Republican for a senator makes sense for conservatives and constitutionalists.
1. FEDERAL COURTS AND SCOTUS The Senate alone has the power to confirm judicial nominees including Supreme Court Justices. If the Democrats control the Senate during the presidency of Donald Trump, there will be zero confirmations of any Trump Supreme Court nominees (except for a flaming Democrat approved liberal) and SCOTUS will become a 4-4 court which absolutely translates to SCOTUS becoming a lame duck court that will be incapable of rolling back unconstitutional acts of Congress and federal agencies. Bear in mind that Obama and the Dems had 8 years to load up the federal court system with liberal jurists. Trump's presidency holds the potential to level the ideological balance of the federal judiciary.
According to the SCOTUS website "Each Term, approximately 7,000-8,000 new cases are filed in the Supreme Court.", and "Plenary review, with oral arguments by attorneys, is currently granted in about 80 of those cases each Term, and the Court typically disposes of about 100 or more cases without plenary review.", here.
Plenary review is a legal procedure defining how higher courts review the decisions of lower courts.
In any event, SCOTUS takes very few cases and just because somebody files an appeal to SCOTUS, the probability of SCOTUS accepting the case is quite low (close to zero).
2. TREATY RATIFICATION AND US SOVEREIGNTY The Senate alone has the power to ratify treaties and it's a difficult process requiring a super majority 2/3 ratification. Consequently, few treaties are ratified. This is critically important because UN and global governance advocates seek to weaken and ultimately destroy US national sovereignty by ratifying UN treaties.
The Republicans have successfully stopped many nasty treaties from being ratified including the Law of the Sea Treaty also know as LOST which would have put all the oceans and seas under UN control. The Republicans have stopped the Democrats from ratifying the UN's Small Arms Treaty which was a direct assault on gun rights.
Republican Senators generally tend to uphold US sovereignty and reject sovereignty slashing UN treaties. The Democrats welcome outsourcing US sovereignty to supranational institutions.
3. SECOND AMENDMENT The R's are far more likely to uphold the 2nd amendment than the Democrats who have a long history of not only opposing the 2nd amendment but slowly hacking away at it.
In summary, the courts, treaty ratification and the 2nd amendment are 3 big reasons to vote Republican in Senate races, especially tight Senate races in swing states.
The Democrats gaining control of the Senate would be an abomination and Chuck Schumer as Senate Majority Leader would derail the entire Trump Agenda. Therefore, anybody who lives in a swing state where the Senate race will be tight, hold your nose and vote Republican because the alternative is far worse.