The Republican Party establishment has campaigned aggressively ahead of Tuesday’s congressional primaries, optimistic that they can not only score a round of victories but also send a message to would-be conservative challengers that 2016 will be another year of electoral setbacks for them.

The incumbent lawmakers and their allied groups would be absolutely confident of victory, too — if it weren’t for Ted Cruz, Donald Trump and the turnout-busting GOP presidential primary.

The first congressional Republican primaries of 2016 feature battles in Texas, Arkansas and Alabama, the last of which also hosts a Senate primary. On a day that could shape the presidential primaries for months to come, the outcome of these intra-party fights will also set the tone for the year’s down-ballot showdowns among Republicans.

And under normal circumstances, the GOP incumbents running to win their party’s nomination, nearly all of whom have run real campaigns backed by big money, would likely be resting easy Tuesday. But the emergence of Trump and Cruz in the presidential primary, which holds concurrent contests in this trio of states, has galvanized outsider sentiment and scrambled turnout predictions.

Now, Republican officials are watching these races cautiously, expecting victory but cognizant that the wild top-of-the-ticket race could throw their carefully laid plans off kilter. Win, and they can shrug off fears that outsiders such as Cruz and Trump will start a wave of upsets across the congressional primaries. Lose — or, more likely in the case of races in Alabama and Texas — fail to avoid runoffs, and the party establishment will brace for a difficult primary season.

“March 1 is an important day to send a clear message: Governing is on the ballot,” said Rob Engstrom, national political director at the U.S. Chamber of Commerce. The chamber, as it did in 2014, has worked diligently this year to support GOP incumbents it has endorsed fend off primary challengers.

Republicans might have the most to worry about in Texas, where a handful of incumbents such as Houston-area congressman Kevin Brady faces a swarm of challengers trying to keep them under the 40-percent threshold necessary to avoid a runoff. In Arkansas, Republican incumbents such as Sen. John Boozman have mostly clear shots at winning their primaries.

But most eyes will be on Alabama, where party officials are closely watching the primaries against Reps. Martha Roby and Bradley Byrne and the day’s biggest showdown — the contest between four challengers and Sen. Richard Shelby. The 81-year-old incumbent, seeking his sixth term in the Senate, needs to win more than 50 percent of the vote to avoid a runoff against likely opponent Jonathan McConnell, a 33-year-old former Marine who has emerged as the senator’s most prominent challenger.

McConnell has tied himself directly to Cruz and Trump, arguing that supporters of the two presidential candidates who vote Tuesday have no reason to back a long-term incumbent who himself is part of the establishment.

“For 37 years, Richard Shelby has reigned in Congress, and for 37 years we’ve only gone further into debt and become less safe,” McConnell said in a statement Monday. “The emergence of anti-establishment candidates like Ted Cruz and Donald Trump has made it abundantly clear that Alabamians are tired of the big government status quo.”

Tying himself to two of the three GOP primary’s leading contenders makes sense, especially in populist-friendly Alabama. Combined, Cruz and Trump receive half or more of the vote in recent polls of Alabama. (One Republican watching the state said polling he had seen suggested Trump alone could reach near 50 percent of the vote.) Also working in his favor: Trump, in particular, has brought scores of new voters into the GOP primary and boosted turnout significantly overall. In the Nevada caucuses, for instance, the reality TV star won the support of more caucus-goers in 2016 than all did all of the 2012 candidates — combined.

But establishment Republicans are buoyed — not just in Alabama but elsewhere in Tuesday’s primaries — at the Republican incumbents’ diligent preparation for the primaries, which they hope will offset any insurgent momentum offered by the presidential race.

In Shelby’s case, at least five outside groups made independent expenditures on his behalf, including the National Rifle Association of America Victory Fund and National Association of Realtors Congressional Fund. One Nation, a GOP-aligned nonprofit organization led by Steven Law, a former top aide to Sen. Mitch McConnell of Kentucky, spent $140,000 on radio ads.

Shelby’s campaign, which has spent more than $8 million since October, has also run an aggressive barrage of TV ads for months and even funded a negative digital campaign aimed at McConnell. It’s the kind of bare-knuckled effort, GOP strategists say, that could insulate him from a potentially tough environment.

“At his age and his tenure, and in this environment, the only way to go after it was like you’re killing snakes,” said Brad Todd, a longtime Republican strategist. “You can’t say after the game, ‘Wow, I wish we had done more.’”

McConnell and his supporters will say Shelby’s aggressive approach is proof he’s running scared. To the senator’s allies, however, he’s merely doing his necessary due diligence.

William Canary, president and CEO of the Alabama Business Council, predicted a “decisive” win for Shelby, which he described as earning 55 percent or more of the vote. Turnout is a factor, he said, but he thinks few of the Trump and Cruz supporters will be receptive to an anti-Shelby message.

How much do the hopefuls running for president have in common with other candidates on the ballot? Canary asked. “I believe strongly, very little.”

Contact Alex Roarty at alexroarty@cqrollcall.com and follow him on Twitter at @Alex_Roarty.