Ahead of election day tomorrow, Republicans are certainly feeling the pressure in judging by how the latest polls are shaping up. Earlier in the day, we've already looked at what's at stake for US president Trump and financial markets. So now, let's take a look at where exactly could Republicans lose their grip on Congress going into the midterms.In essence, these will be the battlegrounds that will be worth keeping an eye on tomorrow.Let's start with the Senate first. Democrats need a net gain of four seats to wrest control from Republicans here. And the latest polling suggests that there are several Republican-held seats facing competitive contests. Here's a quick rundown of that:Besides the above, the other side of the equation is just as important. Democrats too have seats that they must defend, particularly the open contest in Michigan. Otherwise, any Republican losses elsewhere could be offset.And then there's the House, where things get even messier.As of last week's ratings, Cook Political Report mentions that 14 out of 18 toss-up House distrcits are Republican-held and that another 13 Republican-held seats are rated as leaning Democratic. Yowza. Now, that's quite a few districts to keep track of. But if you're going to watch just a few, then I would shortlist these ones:Now, do keep in mind that these are only snapshots of the electoral landscape. Polling leads are not guarantees and the way votes translate into seats can still spring a surprise or two.In any case, the thing that I would be paying attention to is whether Republicans are losing competitive House districts at a faster pace than anticipated and whether Democrats are making gains across several Senate battlegrounds simultaneously.That will be what markets need to be accounting for as the any surprises or sudden shifts from expectations could make for potential repricing flows in Treasury yields and the dollar.All that being said, it is best to remember that tomorrow isn't just about who wins or loses. At the end of the day, it is very much a major test of how much political room Trump will have left to manoeuvre in his remaining tenure as US president. This article was written by Justin Low at investinglive.com.