The last time a Massachusetts Republican won a seat in the U.S. Senate — the only time in the last 40 years, in fact — the fate of Obamacare was said to hinge on the outcome.

Scott Brown won, in part, by pledging to be the vote that would kill it.

That scenario has returned, in dramatic fashion: we just witnessed a Senate roll call in which one more vote would have moved the “repeal and replace” bill forward, likely driving a sizable stake through the heart of the Affordable Care Act (ACA).

Yet, just a few days later, Trump-supporting, friend-of-Brown, Obamacare-opposing Republican Geoff Diehl officially launched his Senate campaign against Elizabeth Warren without drawing any attention to that topic.

Times have changed, apparently.

At his campaign launch last Tuesday in Whitman, Diehl barely mentioned his support for repealing ACA. Coverage of that speech, and his subsequent kickoff tour, focused on his emphasis on job creation, tax reduction, veterans’ affairs, opioid addiction, and immigration.

Diehl has never been shy about the topic in the past.

Just last November, he wrote the “Yes” response for a Boston Globe op-ed exchange on whether ObamaCare should be repealed.

But that argument — or any mention of healthcare — is absent from Diehl’s new Senate campaign website. I also could not find a single mention of the issue in roughly 200 tweets and retweets from his Senate campaign twitter account since his August 1 announcement. He has focused much more on immigration and taxes, and alleging that Warren is an extremist, a partisan, an absentee Senator, and a hypocrite.

Obamacare is the classic dog that didn’t bark, to use the idiom of Sherlock Holmes.

Healthcare is, by far, the biggest issue relating to the U.S. Senate these days. Yet Diehl — like Senate Majority Leader Mitch McConnell—seems more than ready to move on and leave that topic far behind.

Diehl and his campaign spokesperson declined to speak to me about the role of healthcare as an issue in the campaign.

But his silence presumably stems from public polling that shows majority support for keeping Obamacare, and widespread disapproval for the repeal bills considered by Congress this year.

He’s not alone. A scan of campaign websites and news coverage shows that many Republican Senate challengers across the country have kept their heads down on the issue, rather than differentiating themselves from the Democratic incumbents they face.

That might change as more top-tier candidates officially start their campaigns against potentially vulnerable Democrats.

And there are exceptions, such as Luke Messer of Indiana—a current congressman who is already on record voting to repeal the ACA. Messer has continued to talk about repeal in his Senate campaign. Patrick Morrisey, Attorney General of West Virginia, announced his Senate challenge to Democrat Joe Manchin in that heavily pro-Trump state last month, with an emphasis on repealing Obamacare.

Diehl might also come to feel pressure to emphasize his opposition to Obamacare, as the primary contest heats up.

If Obamacare repeal is left unresolved, as increasingly looks to be the case, it figures to be a differentiator among Republican candidates. If Diehl won’t talk about it, others are likely to.

So far, most haven’t. Conservative Allen Waters of Brewster, like Diehl, does not list health care as an issue on his Senate campaign website. Businessman John Kingston, a conservative but also an outspoken Trump critic, has also not staked out a public campaign position. (He and a campaign spokesperson could not be reached for comment.) Beth Lindstrom, a long-time Mitt Romney loyalist considering entering the field, has no public position.

But Shiva Ayyadurai, the wild card technology entrepreneur who could peel pro-Trump primary voters from Diehl, does mention healthcare on his website. Though brief and vague, a blurb there denounces both Obamacare and the House repeal-and-replace bill, calling instead for more “choice, accountability, and affordability.”

And Trump himself might push the issue into play. He and White House officials demanded, prior to the start of the congressional summer recess, that the Senate vote again on healthcare. If they resume that drumbeat when Congress returns in September, it will be difficult for Republican Senate candidates to stay quiet.

Besides, there’s someone else who might make silence difficult: Warren.

Massachusetts incumbent senator doesn’t seem at all shy about raising the topic of ACA repeal—sometimes in rather blunt language that begs for media attention, and opposition response.