It’s not just presidential candidates on the ballot tomorrow: Voters in a handful of states get the first chance to pick House and Senate nominees ahead of November’s elections — and some Republicans are nervous that high turnout generated by the presidential election threatens several incumbents.

Chief among the hand-wringers is Sen. Richard Shelby, who faces four fellow Republicans in tomorrow’s Alabama primary. The long-time incumbent anticipated the prospect of a close election — asĀ we reported back in January, he had booked more than $6 million in television and radio spots, beginning way back in December. His closest challenger, retired Marine Capt. Jonathan McConnell, has been on air too, though at much lower levels.

In Texas, Republican Reps. Louie Gohmert, Kevin Brady, Sam Johnson, John Culberson and Bill Flores all face challengers coming at them from the right. If they don’t hit the 50 percent mark tomorrow, they would be forced into runoffs on May 24.

One thing every Republican strategist agrees upon: Turnout will be unusually high tomorrow. What they don’t agree on is whether incumbents should be most worried about a Trump surge or a Cruz surge.

Cruz voters, on one hand, are more ideologically committed than Trump voters. The entire rationale for Cruz’s campaign is that he can bring out millions of voters who stayed home rather than hold their nose for relative moderates like Mitt Romney and John McCain. If those voters turn out at disproportionately high margins, they are likely to vote against anyone they see as a part of what Cruz calls the Washington Cartel — and that’s bad news for incumbents running for re-election.

On the other hand, Trump voters are so fed up with the system as it stands today that they back a candidate whose entire campaign is about blowing it up and starting new. Those who think Trump helps incumbents say his voters aren’t necessarily well-versed enough on Congress to vote out their incumbents; those who take the opposite view say low-information voters backing Trump will give the boot to any incumbent.

Around the country, any Republican with a legitimate challenger will be watching closely for clues about their own races, later on this year.

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