As Republican primary voters get closer to the polls, it’s beginning to feel like mid-2016 again: Donald Trump and Ted Cruz are on opposing sides.

The former president’s late backing of J.D. Vance in Ohio and Mehmet Oz in Pennsylvania is directly at odds with the Texas senator’s favored candidates in two brutal Senate primaries that conclude next month. Cruz endorsed his candidates before Trump — Josh Mandel in Ohio and David McCormick in Pennsylvania — and didn’t exactly plan to go toe-to-toe with his 2016 presidential rival. But Trump’s late interventions certainly aren’t scaring him off.

In fact, Cruz is dropping into Ohio this weekend to try and push Mandel across the finish line just days before the May 3 primary. And he says he’ll probably head back to Pennsylvania to bolster McCormick against Oz ahead of the May 17 contest.

Trump’s blessing is boosting Vance and Oz, but both states’ Republican primaries are tight enough to be toss-ups. The races amount to a test of whether Trump’s attraction to celebrity candidates can sway the GOP base more than Cruz’s preference for doctrinaire conservatives. And it’s a reminder that before they were allies during Trump’s presidency, the two had very different ideas about the direction of the Republican Party.

“He makes his own endorsement decisions. I had endorsed Mandel well before President Trump got involved,” Cruz said in an interview on Wednesday of his early April endorsement. The Texan added a conciliatory touch, noting that he’s backing “all sorts of candidates all over the country who President Trump is supporting as well.”

Cruz called Mandel the “most conservative and the strongest candidate in the race” but declined to take a shot at Vance or anyone else in the field, calling the wild and open primary race “healthy.” Mandel said Cruz’s weekend swing with him shows Cruz “believes I am the only candidate in this U.S. Senate race who will have the backbone and courage to stand up alongside him in Washington.”

Just a couple of hours later, Cruz endorsed Rep. Ted Budd in North Carolina’s race after spending time last weekend stumping for Adam Laxalt in Nevada. Both Senate candidates also have Trump’s support.

Even so, Cruz’s decision to keep working for his candidates opposite Trump is captivating the GOP. Cruz is making clear he’s not backing down in Pennsylvania either: “I’ve already done a couple of events for Dave and I certainly would anticipate that I may well do more.” Trump is expected to do a rally with Oz next week.

Senate Republicans are watching the party’s upcoming primaries with great interest in what kind of nominees they’ll have to take into the general election. They’re a little curious about Cruz’s and Trump’s role in it, too, given Cruz’s public interest in pursuing another presidential campaign and Trump’s flirtations with a third bid.

“It’ll be an interesting experiment,” said Sen. John Cornyn, Cruz’s Texas GOP colleague. “Obviously, President Trump and Sen. Cruz have history.”

Cruz’s snub of Trump at the 2016 convention and subsequent role as a key Trump backer during his presidency is well-known, but their political relationship has always been complex and situational. In 2015, as the massive GOP primary field began shaping up, Cruz abstained from dinging Trump as the rest of the field criticized him — a strategic move that helped Cruz stay in the race longer but eventually put him on course for an infamous duel with the future winner.

Cruz lost that fight, then initially balked at the expectation that he would endorse Trump. Those days are long behind them: Cruz was a steadfast defender of Trump for his four years in office and helped lead the objections to President Joe Biden’s election certification in 2021. These days the two still regularly discuss politics, with Cruz visiting Trump recently in Florida.

Nonetheless, their electoral competition over the next few weeks is not lost on anyone.

“It’s reflective of their respective personal positions and philosophies,” said Sen. Kevin Cramer (R-N.D.). “They were aligned, but prior to that, they were not. They were rivals. I think it’s pretty cool, it’s pretty healthy.”

Polls indicated Trump’s endorsements of Vance and Oz have been helpful but haven’t shut down the competition altogether. Even when it comes to Trump, “people exaggerate the value of political endorsements,” said retiring Sen. Pat Toomey (R-Pa.), who voted to convict the then-president in his impeachment trial last year for inciting an insurrection at the Capitol.

“I’m not suggesting he doesn’t have any influence. But I suspect people generally, probably overestimate the value of it,” Toomey said. “It’s definitely not dispositive. It’s a boost for Oz, it’s a bump for him but I think it’s still a close, competitive race.”

Toomey said he has no plans to get involved in Pennsylvania‘s primary and believes either McCormick or Oz can win a general election. Cramer and Sen. Lindsey Graham (R-S.C.) said they were not surprised to see Trump back Oz, given their shared roots in the television industry.

In Ohio, retiring Sen. Rob Portman (R-Ohio) will campaign this weekend with former state GOP chair Jane Timken, whom he tried to get Trump to support. But Trump soured on Timken after she defended Rep. Anthony Gonzalez (R-Ohio) following his vote to impeach Trump.

“He came close to endorsing Jane. And then she made the comment about Anthony Gonzalez,” Portman said. “I was pretty sure he was going to endorse. He waited until the end.”

But Trump’s endorsement of Vance, a onetime critic, shocked other Republicans. Mandel had sought aggressively to portray himself as the pro-Trump candidate in the Senate primary, earning Cruz’s endorsement just three weeks ago. Sen. Josh Hawley (R-Mo.) endorsed Vance last September and immediately pitched him to Trump.

“I called Trump and said I’m going to endorse J.D. … he asked me some questions: ‘J.D. said some negative things about me in the past’, so we talked through that,” Hawley said of their conversation last fall. “We hadn’t talked about Ohio in several months; we talked about Missouri. I had thought based on our previous conversations he’d probably stay neutral.”

Cruz has endorsed Missouri Attorney General Eric Schmitt in that state’s Senate race, and Hawley has endorsed Rep. Vicky Hartzler (R-Mo.). Republicans are still somewhat concerned that Trump could endorse scandal-ridden former Gov. Eric Greitens in the crowded primary there, although they also say it’s entirely possible he stays out of it entirely — or that he might endorse someone else.

But if Trump does make a call in Missouri, it will put him at odds with either Cruz or Hawley. And Cruz is certainly not the only Republican who’s ended up opposite Trump in key races. Former Secretary of State Mike Pompeo is siding against his former boss in a contested West Virginia House primary.

And a number of Senate Republicans endorsed Katie Britt, a former aide to retiring Sen. Richard Shelby (R-Ala.), over Rep. Mo Brooks (R-Ala.), whom Trump endorsed early in Alabama’s Senate race before unceremoniously dumping him last month. Cruz also endorsed Brooks.

On Tuesday, all eyes will be on Ohio to see whether Trump can push a once-struggling Vance campaign to victory over Mandel, the early frontrunner. And plenty of Republicans will be watching whether Cruz’s or Trump’s pick comes out on top.

“Trump can elevate you 15-20 points, it seems to me. But then can you sustain it, can you close the deal?” Graham said. “Ted’s endorsement of Mandel may give him just enough of a boost to make it competitive.”