Indiana Republican candidates for U.S. Senate Marlin Stutzman, left, and Todd Young. (Michael Conroy/AP)

Update 8:20 p.m.: As expected, Rep. Todd Young (R-Ind.) easily won his Senate primary with Rep. Marlin Stutzman (R-Ind.) on Tuesday, with Young up 64-36 with 44 percent of precincts reporting. It's the latest evidence that Trump's effect on the GOP primary hasn't really filtered downballot this primary season, as the establishment keeps winning.

The original post follows:

If things go according to plan for Republicans in Tuesday's Indiana GOP Senate primary, this could be one of the last stories you read about the race and its impact on control of the Senate in 2016.

Their preferred candidate, Rep. Todd Young, is expected to win the primary over a hard-line conservative with tea party support, Rep. Marlin Stutzman. Young, a former Marine, would then be in a good position to win the open seat being vacated by retiring Sen. Dan Coats (R) and keep the seat Republican.

That scenario seems much more likely Tuesday than it did a few months ago. Back then, we were analyzing whether GOP's preferred Senate candidates were in trouble because they were sharing the ballot with Donald Trump, who is most definitely not the GOP establishment's preferred presidential candidate.

But in the five GOP Senate primaries that have followed, we've seen basically zero evidence that Trump's victories are bad news for Republicans' top congressional primary candidates. Just because millions of voters are won over by Trump's outsider appeal doesn't mean they'll automatically vote for any candidate playing up their outsider credentials or attaching themselves to Trump.

In other words: The Trump Effect seems pretty difficult to replicate if you're not a once-in-a-lifetime, incredibly unique candidate named Donald Trump. And there haven't been outside GOP candidates this primary season who remotely resemble Trump.

Thanks in part to that, Senate Republicans are actually having a better primary year than in some recent cycles. They're 5-for-5 in contested primaries so far even though Trump has won 4 out of those 5 states, often by large margins. In 2010 and 2012, the Republican establishment lost at least four primaries each cycle.

They had prepared to do battle this year, too. In March, longtime senator Richard Shelby (Ala.) became the first congressional incumbent to test out being on a ballot with Trump.

He took nothing for granted. Shelby spent at least $5 million in his primary to avoid having a runoff with a little-known challenger. He won and, more important, avoided a runoff by taking a majority of the vote in a crowded field, even as Trump won his state by more than 20 points. It was a silver lining in what was otherwise a terrible Super Tuesday for the GOP establishment.

But at what cost did Shelby's win come, I wondered at the time. Not every Senate incumbent or establishment-preferred candidate can spend $5 million to keep from getting booted in the Year of Trump.

Turns out they haven't necessarily needed to. In Illinois, 20-year veteran Rep. John Shimkus (R) successfully faced down a tea party primary challenger who had the backing of the conservative Club for Growth. The closest the establishment has come to a scare from an outside challenger was Rep. Bill Shuster's (R-Pa.) April 26 primary, where an underfunded and little-known political novice came within a point of knocking off the powerful House transportation committee chairman. So far though, the only House incumbent to lose a primary this year has been a Democrat (and he's under indictment).

The establishment is very much expected to stay flawless Tuesday; in Indiana, the most recent Wall Street Journal/NBC News/Marist poll shows Young is ahead of Stutzman by 32 points even though Trump is expected to win the state.

There are a few reasons the Trump Effect hasn't panned out in GOP primaries. For one, the tea party and Trump aren't one and the same. The group that tends to support these outside candidates, the Club for Growth, is actively spending against Trump at the presidential level. In other words, not only aren't there a lot of candidates in the Trump mold; there isn't really an outside group geared toward promoting such candidates.

The Trump Effect (or lack therof) aside, Republicans have also gotten lucky this cycle by avoiding a ton of substantive primary challengers. The Club for Growth has only backed two challengers so far, in part because the other candidates just aren't viable, its spokesman, Doug Sachtleben, told Roll Call's Simone Pathe. Their only congressional win came in an open Ohio House seat to replace former House speaker John Boehner . Hugely symbolic, perhaps, but not much to extrapolate there about Trump's effect on down-ballot primaries — especially in a very crowded field.

In Indiana, the Club for Growth had originally given some support to Stutzman — until he threw out his D.C. campaign team for a local one. And Stutzman is facing damning Associated Press headlines about billing his campaign for a family trip and paying his brother-in-law $170,000 for campaign work. (Stutzman's campaign has denied any wrongdoing.) Stutzman, of course, wasn't really supposed to be a particularly strong candidate; he had already lost a 2010 primary to Coats.

In Alabama, Shelby was facing a bunch of untested opponents, such as 33-year-old Iraq War veteran Jonathan McConnell, who wasn't well known outside of his father running Alabama's Republican Party at one time. In Ohio, Sen. Rob Portman (R) faced off against a 65-year-old retiree who lost a 2008 congressional race as an independent. Portman got 82 percent of the vote. We could go on.

Republicans say they're not just coasting on luck; they're actively making sure no challenger catches on with Trump voters. In Indiana, for example, Young's campaign started a website that describes Stutzman as a career politician, among other things anathema to Trump supporters. Outside groups allied with Senate Majority Leader Mitch McConnell have spent millions of dollars to boost Young and attack Stutzman. 

Preparation and luck have helped congressional Republicans avoid a primary catastrophe this year — along with the complete uniqueness of the man shaking up the GOP's political world.

But Republicans can't get too cozy with their success sharing a ballot with Trump.

A very different Trump Effect could be very real in the fall if Trump is the party's presidential nominee. Senate Democrats have a shot at taking back the Senate, thanks in part to their plan to tie all Republicans on the ballot to the deeply unpopular Trump.

So far in the primaries, though, it seems people aren't looking for — or at least haven't found — the congressional version of Trump. And that's good enough for the GOP establishment — for now.