The battle for the Senate in 2018 is caught between two opposing forces: math and President Trump.

Let's start with math. Senate Democrats have a heckuva challenge defending their lawmakers in the 2018 midterm election: By virtue of their 2012 victories in some swing and red-leaning states, they now have to defend 25 seats, 10 of which are in states that Trump won.

In some states, like the one Sen. Joe Manchin III (D-W.Va.) is trying to win reelection in, Trump beat Hillary Clinton by more than 40 points! By comparison, Republicans only have to defend nine seats, one or two of which could be considered vulnerable.

It's feasible that Republicans could expand their 52-seat majority, and, if they had a near-perfect run, get to the coveted filibuster-proof 60.

On the other hand, you have Trump. The party in power normally gets blowback in the first midterm election of a new president. And this president is at historically low approval ratings this early on, with warning signs that traditional GOP voters aren't thrilled with his and Congress's performance so far.

Here's another potential warning sign for Senate Republicans that Trump's shadow could undermine their position of strength: Some top potential Senate candidates are turning down the opportunity to challenge vulnerable Senate Democrats.

In Pennsylvania, four-term Rep. Patrick Meehan (R) was considering, then declined, to challenge Sen. Bob Casey. Meehan would have been a bigger name than the two state lawmakers and one borough councilman who have jumped in so far to try to challenge Casey.

In Indiana, a state Trump won by 19 points, Rep. Susan Brooks (R) said she wouldn't try to challenge Sen. Joe Donnelly (D). The IndyStar said Brooks would have been a “potentially formidable opponent,” though it reports two other GOP House lawmakers are considering a run as well: Reps. Luke Messer and Todd Rokita.

In Wisconsin, a state Trump won by less than a percentage point, leading potential challenger Rep. Sean P. Duffy (R) said he won't run against Sen. Tammy Baldwin (D). “This is not the right time to run for Senate,” he said in a statement, pointing to his eight kids. A couple of state lawmakers, a teacher, a Marine veteran and a businessman are all considering running, which could create a messy primary.

Republicans' recruitment struggles in Montana is related to Trump but in a different way. Former representative Ryan Zinke (R) was thought to be Republicans' strongest candidate to challenge one of the most vulnerable Democratic senators, Jon Tester, in a state Trump won by 29 points. Then Trump picked Zinke to be secretary of the interior, and it isn't clear who will challenge Tester beside a first-term state senator who recently announced he's in.

November 2018 is still a year and a half away, so there's no rule that Senate candidates have to get in right now. But already, several potential top-tier candidates in Trump states have thought about challenging Democrats, then decided not to. That doesn't help Republicans counter a nascent narrative, both in GOP circles and outside of it, that Trump could weigh them down in 2018.

This is all playing out in the context of Democrats flush with momentum and money from a liberal base stoked to challenge Trump. Many of these Senate Democrats reported this week that they raised a record amount of money for their states a year and a half before the election. (Though Republican Sen. Dean Heller of Nevada kept pace with them, too.)

And a closer-than-expected congressional election in a red district in Kansas and a coming one in Georgia suggest that voters in traditional Republican districts aren't thrilled with their party's performance in Washington so far.

Of course, Republicans have more opportunities to knock off Democrats than just these couple of states we listed. And Democrats, who only have two-ish Republican states where they can feasibly play in, don't have candidates yet either.

But since we're going to spend the next 574 days trying to assess which opposing force is stronger in the 2018 Senate midterms — math or Trump's unpopularity — let's plant an early flag and say that, so far, Trump's unpopularity appears to be weighing on Senate Republicans.